Where to buy:
There are new some versions on Amazon.com (Thanks Natasha!). They also cycle in and out of toy stores in various versions. Keep checking the official site for information on the latest releases.

Sept. 9, 2006: I found these at Big Lots, only $9 and $4! And they have the 'PP' emblem embroidered on the back, which should really be required. They had several colors in both sizes. If you have one nearby, it might be worth a stop if you're looking for the newer ones. I keep expecting them to show up at stores like Dollar General, but no luck yet.

For the classics: There's always Ebay, but also try local flea markets or even antique stores. I've bought several for only $1 each that way.

Picture/information sources:
PoundPuppies.com - official site
Yesterdayland
megsplace.com
TimelessTrinkets.com
The current layout image was scanned from my own 1988 Pound Puppies calendar that I found while cleaning my room. :-)