bolts meant to hold two rings to the crank arms will be slightly longer than track bolts, which are meant to hold a single ring to the crank arms.
there could be exceptions to this (thinner spider, thinner rings), but in my experience, every double/triple i've converted required shorter bolts.

yes, they will have them. any shop (even big chains) will most likely have them. a shop specializing in tarck culture will undoubtedly carry different types, colors, materials, etc.