Taylor Greenhouses carries it. I think Kartuz does too. You could check with Palm Hammock Orchid Estates as well.

If you are a member of the ABS you could probably buy some seed from them.

This begonia is not an easy one to grow - can grow very tall and loves it outdoors. Thiemei or carolineifolia would be a lot easier or even hybrids of luxurians such as Thompson's Folly, Freda's Folly, Lady Brown (great for summer but not happy in winter - takes after its parent), Mike Flaherty, Paul Hernandez (this one gets HUGE), and many more.

Annie's Annuals in Richmond, CA also sells them, but their shipping can be steep outside the West coast unless you buy a lot of stuff (but all their stuff is good...).

Strybing Arboretum in Golden Gate Park, San Francisco, sells them for a good price at their monthly sales (next one is Saturday at 10:00 A.M.), but they typically sell just one, I think.

I was just told that B. luxurians was featured on Martha Stewart, so I would expect that it will become hard to find.

Here is a link that might be useful: Begoinia luxurians at Annie's Annuals