PT draws down really nice! Remember you don't need to flux it when you heat it for annealing purposes
Pretty much anyone you can order precious metals from would be a good source. stuller, hoover and strong, rio grande, united PMR, johston matthey, ect...
Pretty much all the pt in the US originates from JM at some time or another. There's really only three alloys that are commonly available. 900pt/iridium, 950pt/ruthenium, and 950pt/cobalt. I'd stay clear of the cobalt alloy. I use the ruthenium alloy exclusively
what are you planning to do with it? if you're going to be inlaying it like you would 24kt gold wire, get PURE (.999?) platinum. i get it from Hoover & Strong.