Not sure if HD would carry square tubing like that (don't recall seeing it there).
Check your yellow pages for steel suppliers. They should be able to sell you what you need. Also, I'm guessing they went with 1.25" square tubing to use the larger bore Olympic plates? You could probably substitute a piece of 1.5" steel ("black iron") pipe, and you would definitely be able to find that at a Lowes or HD.
(FYI, 1.25" square tubing is 1.77 across the diagonal. 1.25" schedule 40 pipe is 1.66" diameter, and 1.5" schedule 40 pipe is 1.9" diameter)
Another good drag item you might consider is fire hose. Not sure if they'd do it, but you could ask your local fire department to save you their 2.5" diameter hoses when they wear out. Pinch off one end, and fill it with sand, throw it over your shoulder and drag it behind you. Trust me, they're heavy with water in them, I'm pretty sure they'd be just as bad full of sand!! :hmm: