Hit-type cameras usually use 17.5mm film. A few use 16mm film. The 17.5mm film comes from splitting unperforated 35mm film into two strips.
So with these cameras you have four options.
First, you can try to find 35mm unperforated film and split it yourself. Building a film splitter is possible, but the available 35mm unperforated film is quite limited.
A second option is to split perforated 35mm film. You'll end up with a large selection of film, but you'll still have to split it and a large chunk of the image will fall on the film perforations and not be useable. Your final useable image will not be 14x14mm, but closer to 14x10mm.
Your third option is to use a larger film size, such as 35mm, and slit a 17.5mm section out of the middle. This removes the perforations, but you only get one strip of film from 35mm film instead of two.
Another option is to use 16mm film. It's super easy since no cutting is needed, but the film may slide around a little inside the camera and you might end up with a slightly cut-off image. And if you use 16mm film with any perorations (single or double), your image size will be substantially reduced.
No matter which film option you select, HIT film needs a paper backing to prevent the film from being exposed through the frame counter window on the camera back. Some people opt to cover up the window with black tape. The advantage of this approach is that you can load the film directly into the camera without the nuisance of paper-backing -- and you can load more film as a result. The down-side to this is that the camera must be loaded in complete darkness -- NOT an easy task.
If you prefer the standard approach, you can substitute the paper backing from a 110 cassette for the HIT paper, if you need. But be aware that you increase the likelihood of light leaks. HIT paper-backing is 17.5mm wide while the 110 paper is only 16mm wide. Light can leak around the edges. In addition, the numbers on the 110 paper are COMPLETELY different from the 17.5mm numbers -- DON'T USE THEM! (if all else fails you can cut a strip of paper from a 120 roll of film -- 17.5mm wide and 18 inches long) But no matter what type of paper-backing you use, make a mark on the paper-back where you attach the film. It should be about an inch (or less) before the "1" number on the paper. Carefully wind the film (and paper) onto the film-feeder spool. If you are not using an original 17.5mm paper-backing, write the numbers 1 through 10 16mm apart on the paper so that you can see them through the counter window.
Hit cameras usually don't have film counters but were designed for 10 exposures on about 7.5 inches of film. You should make your film strip slightly longer -- about 9 inches (not to get more exposures, but to make sure your exposures are on the film!)
In any event, with most 17.5mm cameras you will only be able to use the camera if it has two identical film spools. Most used HIT cameras are bought with only one spool, but you'll probably need another reel to be able to reload the camera successfully. The HIT spools look just like tiny 120 film spools -- one for each side of the camera. In addition, nearly all HIT cameras had a spool holder for the film-feeder spool -- or a double-reel holder for both reels. This is not an inconsequential item -- it makes loading the camera a LOT easier. Without it the film tends to unroll all over the place when you try to load the camera. The holder has a tab that keeps pressure on the film roll and keeps it from unraveling.
To load a reel, start by attaching one end of the paper-backing to the feeder spool. Don't tape it to the reel, since the backing must be able to detach from the reel at the end of the roll. Wind up about 4 inches of backing onto the reel. Then turn off the lights. Now place the film on top of the paper backing and start to roll the film-paper combination into the reel. Make sure that the film and paper are aligned. When you get to the end of the film, attach it to the paper backing with a small piece of tape. This will make sure that the film advances correctly in the camera. You should have about 4 inches of backing left at this point. Continue to roll up the backing on the reel.
When you have the film (and paper-backing) wound onto the film-feeder spool, tape it up so that it won't unravel. When you are ready to load the camera, undo the tape and slide the film and reel onto the film reel holder. The good news is that you can leave the lights on, unless you are loading the camera without the film reel holder. Next, attach the end of the paper-backing to the take-up spool. This can be tricky since the leader can slip out easily. Either rotate the reel a few times or tape the leader to the reel to be sure it stays in place. Then slide the take-up spool into the camera. Some spools only have a slot on one end. Make sure there is a slot on the top side of the reel and that it engages correctly with the tab on the film advance knob. This can be tricky. You'll have to make sure that the notched end of the spool engages the gear in the camera. If you try to put the spool in upside down, it won't fit correctly since the spool only has the notch on one end. Then turn the film advance wheel a couple of turns to make sure the spool is fully engaged. If you have the film-feeder spool holder, slip the feeder spool into the holder and slide it into the other side of the camera. The notch on the feeder spool can face up or down.
Close the door and advance the film until the "1" appears in the window and start shooting. If you are not using the paper-back numbers, advance the film at least 16mm for each exposure.
And if all else fails, 17.5mm film will fit on a 16mm developing reel -- with a little force.