Home depot doesn't seem to show it online. They only have 3 liner kits, all for larger ponds, none of them rigid. At least, in their watergarden section
I would second checking out a home depot or lowes. They have a lot of stuff in the stores that is not online (and vice versa), give em call.
Also check out local landscape supply places, some sell pond stuff and know nothing about it but some are really great! My local landscape place (Brock Farms) had a huge sale at the end of the season and they still have some stuff, might get lucky. Also a 5x5 piece of rubber liner is $20, a little too small for a 3' round X 2' deep but darn close!
If you are handy at all there are MANY DIY projects out there. Here is a diy waterfall box for $20, youtube.