It took me a very long time to find mine. My first one was a trade in at my LFS and I just happened to come by before anyone else could snatch it up and I paid a high price for it. The second one was a purchase off of FB from Aquatic Collections. The others I flew out to Los Angeles where a hobbyist had gone to a wholesaler for me and grabbed a couple for me.
That being said, it's insanely hard to find these guys, and rightfully so. They are a species that really should be left to experts and their import SHOULD be limited since they don't replicate very quickly or can be propagated in captivity. They also get big.... like REALLY big. Hence the name gigantea. I've seen some in the wild that were larger than my arm stretched out. Best of luck in finding one, but I urge to consider a different species first.