Not knowing your design I cannot say for sure, but some things to think of...
#1 really think about your design, can you use normal 1/2" round bar and just turn the ends down where the bearings are to size? Further do you really need 7075? 6061 does pretty good for most uses.
#2 that much AL weighs about 50LBS
Could you turn tube down to the proper size?
You may try igus, they make AL shafting meant to fit their plastic bearings (they work really well) website here
http://www.igus.com/drylins.asp They list their shafting as .5 however I couldn't find true dimensions. They have a really nice program where you can get their stuff for free for school projects, they just ask you send them a photo and a writeup of what you did with it.
Good luck