Hobart 140 to start 'cause I didn't have 220 to plug in a new HF 220 unit that I got as a gift. The limitations of the Hobart are that it won't do aluminum (you'll want to do that someday) and it is pretty much maxed out at 1/4" steel (which you probably won't worry about on this project).
That HF unit will probably go bye-bye someday. Seriously, even HF disavows its existence now. It's never been used....
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MetalsDepot.com for mail order has worked good for me. Great for all the special weird stuff.
www.metalsupermarkets.com has a local store that is really great -- scrap pile and access to a huge metal brake Oand people who don't think you're a bother if you ask them to cut a 20' section of tube in two (so long as it's not long-ways?).
I started out with Alro 'cause I lived near their headquarters in MI. They had a retail store there w/ cut offs, etc. I was gravely disappointed when I got to Louisville and they didn't want to deal with my piddly little orders (seriously guys, $120 for 2x20 tubes?). But they also were the one who turned me on to Metal Supermarket down the road (I took the hints).
I used Dillon Supply in Raleigh. Nice people.
If that doesn't work, go to your local circle track and ask the guys in the paddock who they recommend. Or better yet, join your local SCCA and ask those folks.
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Wear gloves when you pick up your steel -- Dirty and sharp edges. Bring a red "flag" cause it's gonna hang out of whatever you're driving. And of course, some ropes/straps. Unless you're sure that your store cuts the 20' tubes for no extra cost (check on that) then bring a hack saw or battery-operated sawz-all.