The plush are relatively easy to find. It didn't take me too long to locate them all. The PVCs took much longer to find.
There was also a set of bedsheets made that I totally had and slept on in college. Heh. And if you look on Youtube, you can probably find the brief cartoon featuring them. It's kinda scary!
I grew up with the Letter People, who never got toys. I remember one of my teachers having inflatable ones, but they weren't on the regular market that I knew of. So I was happy to randomly discover the Alpha Critters and finally have some goofy letter-themed toys.
If you're looking for Alpha Critters, ebay is your best bet. I got most of mine from there. There are loads of reasonable plush on there right now. If you want the PVCs though, you'll have to be patient, because for all the plush listed, there's only one auction for PVCs.
I actually prefer the PVCs because they show the shapes of the letters better. I wouldn't part with either of my collections though. Not yet anyway!
PHOTO CREDITS: Mine.