the best type of thing to look for is toys. honestly thats the best I can tell you. look for anal beads. it looks like a string of rubber beads. the sort of thing that was designed for that. just be sure to buy lube also.
sorry can't advise you on that, i am not into that type of thing.
what type of object should be best for anal masurbation?
Masterbation is safe.
Anal masterbation by self is safe but you might get some bleeding from a foreign object being in the anal cavity.