I had a minute to peruse the web. A lot of these places will also have tropical tree species that might be good for indoor in winter bonsai too, but all I listed carry Monstera or other Philodendron type Aroids.
I've dealt with Top Tropicals, they tend to be on the slightly higher end of the price range, and they charge a lot for shipping, but they often have species that are otherwise impossible to find. I wager they are a really great nursery to visit in person, if you ever get near Fort Meyers area of Florida. They have big and small aroids, including Monstera and Rhaphidophora.
toptropicals.com
Zone 9 Tropicals - Houston TX, mail order nursery - has a few monstera listed under Aroids. They have been around a while, I have not bought from them but would try them without hesitation.
Zone 9 Tropicals fragrant rare plants for home and garden
www.zone9tropicals.com
Gardino Nursery - Florida based - I have done business with them, good, they deliver what they promise. I would order from them again.
Pistil Nursery - Portland Oregon based, I know nothing about them. Just stumbled across their site.
Raphidophora tetrasperma is a fast growing vining plant, sought after for its unique fenestrated leaves. Sometimes called “Dwarf Monstera” or “Monstera Ginny” due to is resemblance to Monstera deliciosa, the leaves of Raphidophora tetrasperma are similarly split with “windows,” but stay quite...
pistilsnursery.com
Ken's Philodendrons - mail order out of Florida - currently "closed for propagation" will open sometime autumn 2020, claims to be "largest" collection of rare Philodendron and related species in USA. May well be, just a glance showed many species I never heard of. Might be worth shopping to find the really rare one.
kensphilodendrons.com
Hirt's garden - an Ohio based nursery, they do pretty good mail order - I have ordered from them and was happy with service and product.
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hirts.com