Is there a plumeria nursery close to you? If so, they might order things like perlite in bulk. If they won't sell it to you they might be able to tell you where they get it from. If there's another serious hobbiest close to you, they have probably figured out a source for their supplies.

I have been experimenting will all sorts of mixes. Currently I'm using a product from lowes called Hapi Gro landscapers mix. It is basically pine bark fines like in Al's 5.1.1. I add 1-8 quart bag of perlite to a 2 CF bag of the mix. So far its doing what I want it to do. Does get hydrophobic if I let it dry out too much.

Brads buds and blooms is in Orlando and Exotic plumeria is in Tampa. You should be able to go to any good nursery and find bigger bags of perlite. Also any Fafard heavy weight mix is pretty good in a bag mix. I had luck using Fafard 52. Its just not cheap here.
Mije