Those are the Zilla "Creature Cubbyhole".
Check the various online herp supply shops, they go for about $2-$5 each plus shipping.
reptiledirect.com has just the large currently, reptilesupply.com has just a smaller size. Herpsupplies.com has the large and extra large.
Wow looks like nobody has all of the sizes any more.
They're nice simple hides and easily cleaned, at one point I was trying to supply every rack shelf with 2 of them, but it got too hard to get enough in the proper sizes as the snakes grew.
Another nice cheap plastic hide, more easily found, is plastic plant saucers from Walmart or a hardware superstore. Cut out an entry hole, either in the top or side, and they're ready to go. Cost $.47 to maybe $1.50 for the largest size.