Sea star is the better term since asteroids (i.e. sea stars) are not, strictly speaking, fish. Species of several genera of asteriods which have been extensively worked upon regarding regeneration of sea star arms and bodies include Linckia, Coscinasterias, and Asterias. Species within these genera should provide you with working material to studying asteriod regeneration. In addition, you should search both the web and the data base Zoological Record for references to rates of regeneration in sea stars, and echinoderms in general. Running "Biological Supply" on "Google" will turn up a listing of supply houses which might suit your needs. A favorite of mine which I have used over the years is "Gulf Specimen and Marine Laboratory" located in Panacea, Florida. Woods Hole Oceanographic Institute has in past sold specimens to students and researchers. You might do better yet to find a university marine station which has a 10-12 week summer program where you would have access to sea stars, biology professors, marine aquaria and library facilities to help you carry out your experiments.