there are a ton of pearl powders (mica powder) on ebay for pretty good prices, in any color you can imagine. color shifting is usually more expensive, but you can get small quantities (like 5 grams) for pretty cheap to try out. I haven't used any color shifts myself (seems to be more to catch the anglers than fish and I don't sell my stuff) but the normal micas work great and you can make some really nice stuff with any of them.
a lot of the places that sell micas also sell various glitters too, from .004 to .040 and some string too, and it seems to hold up to the temps involved in soft plastics.