Go to the 'Care Articles' and read over the pixie frog care sheet written by John. Specifically under the 'Housing' section to get an idea of what is recommended during the growth cycle of raising your pixie. Then you can go from there and pick up a Sterilite or Rubbermaid bin that is suitable for housing your pixie at a Walmart, etc.
This guide is definitely what you're looking for to save money on setting up a plastic enclosure: http://www.frogforum.net/vivarium-terrarium-enclosure-discussion/703-quick-cheap-easy-terrarium-vivarium.html
As far as buying from a big chain pet store such as the ones you listed, I wouldn't recommend it. Usually they tend to be non experienced with raising reptiles and the reptiles they are housing may be sick or weak. Here's a few sites that are far more experienced with reptiles/amphibians and reputable:
Reptile Supplies, Reptiles, Amphibians, Invertebrates, Cages, Feeders, and More
Jelas Phat Phibs | Phibs, Beardies and Feeder Roaches
Josh's Frogs - Largest online herps feeders and reptile supplies store
You wont be disappointed...
Extra Note: Those large chain pet stores selling 'African Bullfrogs' may be a dwarf and just trying to make a quick buck off non-experienced frog hobbyists. So, chances of being a true South African Bullfrog and not a Tanzanian Dwarf Bullfrog is low. Although I could be wrong, but that's what I've seen from shops.
See the difference? May not be the best pictures, but dwarfs usually have a lot of yellow on the underside and a tan line that goes right down their back. The true pixies have green stripes on their backs as babies and are mostly green with some brown that eventually turns into a olive greenish as they reach their adult stage and lose the stripe on their back, unless they're female which I've heard they sometimes keep the green stripe on their back.
Tanzanian Dwarf Bullfrog:
South African Bullfrog:![]()