If Wal-Mart doesn't have what you're looking for (they should, I think what you want are the under-bed storage boxes.), then give a Menards, Lowes, or Home Depot a shot.
I personally prefer Sterilite size and price wise. I use the 15 and 32 Qt. tubs for my racks, but the 32's are only approximately 25" long which isn't what you are looking for.
D80