Joe,
McMaster and Grainger have crummy selections on raw materials. I would personally order from an actual metal supplier. One suggestion is speedy metals (speedymetals.com). Here's the page with 7" CRS:
http://www.speedymetals.com/c-8213-1018.aspx?pagenum=4
Your other route might work too. Unevenness isn't really a problem. The only thing is you want a 1/2" overhang from the steel past the aluminum to set the spacing for the R&P correctly.
Ahren
www.cncrouterparts.com