Where can I find those little rubber shim strips?
 

You know the ones that come extra with a bike light mount or maybe a GPS mount. They’re the really thin ones that are kinda grippy on one side. I don’t know exactly what they’re called and searching rubber shim didn’t seem to help me.

anyone know?

Accessory bracket shims. Try a LBS and they will likely have a bucket full. Andy

As above, or google on bicycle plastic shim strips for mounts for some options. If you have been around the block
a few times you will have a bunch of these from prior computer/rear light/bar light or other accessories laying around,
so you could check with fellow cyclists.

I used to have a bunch but I think they went out with the debris after Hurricane Harvey flooding. Haven’t bought anything that uses them since I guess!

I use pieces of innertube.

I was going to say, they are called innertubes

This.

This this
For years I've kept a couple of knackered tubes hanging on a nail on the garage wall for easy snipping off a bit if needed.

ha... great minds think alike... plenty of rubber shims after changing just 1 flat! :-)

I don't get many flats but I wish I had kept some dead tubes now. Will try to remember that next time I get a flat.

But, is there no place to find the strips by themselves besides asking a bike shop to part with a few?
Hard to even find pics of these things but found a few

https://cimg0.ibsrv.net/gimg/bikefor...b89438eb29.jpg
https://cimg1.ibsrv.net/gimg/bikefor...57b3dc32ab.jpg

I use old innertubes for a variety of things, including the aforementioned shims, as bungee straps, and around the pieces of chain that I use for locking up my bikes. It's a wonder material.

I have a bunch of them in a bin. Can you tell me the diameter of the clamp and the width for clamping, I can send you some even if you are not Bill Nye the Science Guy?

In addition to inner tubes, I buy rubber or vinyl tubing at the local hardware store by the foot. For example, you could buy a foot of 1" diameter, thin wall vinyl tubing. You can cut the tubing to length, then slit/trim the tubing to fit the required diameter. The cut tubing has a natural curl, so it tends to self-wrap. A foot maybe costs $1 at most, and it will last a long time! In fact, I have a handful of short pieces of tubing of varying diameter and wall thicknesses in my garage. Like innertubes, they do come in handy for all sorts or things around the house.

I have a roll of self-fusing silicone tape. Once you clamp it, it won't come off unless you cut it. Getting the thickness right is a little tricky, so better to cut it too small and add to it than to start out too thick. I've really fallen in love with this stuff, because my shims stay where I put them.