You should be able to buy feeler gauges and shim stock at any industrial tooling store. If there isn't one near you, you might consider McMaster Carr.
https://www.mcmaster.com/feeler-gauges I often cut a feeler gauge down to suit a specific purpose, or, if you use shim stock you have even more options in regard to shapes. For ring fitting, this is not really needed. If you cut a gauge, be aware that you may have changed the edge thickness. I keep a plastic gauge set on hand, but these are much softer, so depending on what you are doing, they may be less suitable. In the smaller sizes, they are floppy, which is not an asset for ring fitting.
Fitting a ring properly has a lot of "feel" to it, so it can be hard to describe. Here are a few of the steps:
Take the ring and compress it until you can slide it into the liner, which has been removed from the engine. Square up the ring in the bore using the piston. Remove the piston. Backlight the liner so you can look for fit and gapping. Carefully tilt the liner around in front of your light source until you are observing the gaps (if any) at their largest. You may be surprised at how big they appear, but know that using light gaps is a very sensitive method.
For a conventional ring you can estimate the fit of the thickness before attempting to install the ring. Just put the outer edge into the gap. If the piston has carbon built up in the gap, you must remove this before you start. Most guys will tell you that you should use a broken ring to do this. If you want to try this, I suggest that after you break the old ring, grind the end flat, and at 90 degrees to the local curvature. Don't break the edges after grinding. I would cook the piston in antifreeze first to remove as much carbon without likelihood of damage as possible. It also softens any remaining carbon. I have also used thin basswood strips and baking soda to work on the bottom of the groove. Get the stick wet and dip it in the baking soda.Work the stick back and forth in the groove. The overriding concern is not to bugger up the ring groove unless you want to buy a new piston.
Once the piston is clean, and assuming the groove is still nice and square and not gouged on the bottom, you can think about lapping the ring height if it is needed. If it is already too loose, then go get another ring. If you have not done a lot of lapping before, go research the do's and don'ts. A lapping plate is not mandatory, but something very flat, cleanable and chargeable is. I would make a cylindrical ring holder that has a shoulder and fits your ring. Otherwise, you are just winging it. If you don't have a micrometer to measure the ring as you go, you are also winging it. A good way to measure the piston ring groove width is with a set of gauge pins, but those are expensive and most people won't have any around. I don't. With a full set of shims/feeler gauges, you can stack them to measure the gap. Be aware that the accuracy is less because of the stackup. Don't force steel shims into the groove and bugger it up. This is where plastic shims are an advantage. Don't just trust the cited thickness of the shim stack, adding them all up. Measure the stack with the micrometer.
If you are planning on cutting a pin relief with the Dremel, then you are going to need to be an artist. I have not done this, but suggest you consider clamping the Dremel tool down above a table saw miter groove. Make a sliding plate that you can clamp the ring to. Set the heights and position. You might only get one shot at this. I would use a full clamping pad above the ring. Be sure the ring is undeformed before and after you clamp it. If your saw's miter slot is loose, tighten it up first. Another similar problem is going to be the (very) large endplay of a Dremel. The position may or may not be repeatable at speed. Depending on the size of the pin, you are likely going to have to use the smaller, unreinforced wheel due to its thinner kerf. However it may not be wide enough to do this on one pass. I would practice and fine tune the whole setup using a surrogate ring made out of similar diameter piano wire.
Just a few of the steps. See what other guys do.
Decide from what you see when you look at your parts what needs to be done. Don't expect miracles from old parts. Certainly not from damaged parts....
Good luck,
Dave PS--I forgot to mention that I would measure the gap with the ring at or near TDC. It is hottest up there, and if your liner has taper you need to account for that as well.