For shims I use feeler gauges since they're cheap, precise, come in an assortment of sizes, and have the size written on the shim. I usually use this set from Amazon with starts at 0.0015" (0.038mm) :
https://www.amazon.com/dp/B0009OMY9C
If I need larger or thinner shims I'll go with this assortment pack which goes down to 0.001":
https://www.amazon.com/dp/B00065UXD8
Aluminum foil also makes good shim stock with standard aluminum foil being 0.0006" while heavy duty aluminum foil is 0.0008".
I always measure my shims before using them since I've found they're sometimes mislabeled.
There isn't much reason to be worried about galvanic corrosion between aluminum shims and cast iron, and the potential between them is extremely low. Aluminum foil is approximately 0.90V while cast iron is approximately 0.85V. Even harsh environments allow a difference up to 0.15V, with a temperature controlled environment allowing up to 0.5V.
However, I'd recommend against using stainless steel shims, since they have a significantly greater potential of corroding steel or cast iron.
https://www.engineersedge.com/galvanic_capatability.htm
