where can i get some strike plate material from that tells you the thickness, in other words how do i know if im getting a thinner or thicker side plate material for tuning?
Digital calipers would measure anything and be useful for more than just side plates. As for tuning most just work it until it works. There's not really a set formula that lets you pick/calculate a thickness, order it, and it be perfect. You just have to tinker.
Here is what I do. I use masking tape and paper matches until I get a thickness that works. Then I cement a leather pad with as much backing in a 1/8" wide strip under the leather to get the thickness I need and a central contact point over the center of the arrow contact point (the piviot point usually being rearward of the center of the shelf).
If I don't need much I use a leather with adhesive on the back. Thicker I use leather and two-sided tape. I use the kind R/C pilots use to attach servos. Strong & waterproof.
how do you get the thickness measurement after using the tape or matches, or are you saying you glue over the tape or matches?
A caliper would do the trick. They can be purchased at a reasonable price from hardware stores.