Dave, I got mine originally from McLure & Zimmerman www.mzbulb.com But I just went to their site to check and it seems they've sold out of the red/yellow one (Gloriosa rothschildiana). They do still have the all-yellow flowered ones, and interestingly, a purple one that I've never noticed before. (may have to try it!)
They're pretty commonly available, and I've even seen them in net bags at the big box store a few times. Even Van Bourgondien has them listed, and pretty inexpensive, too, but I haven't been too pleased with their bulbs when it's late in the season like this.
http://www.dutchbulbs.com/prod... IF you order from them, the bulbs should be white or light colored, fleshy and firm. Don't accept them if they're soft or brown.