How about this one?:
http://cgi.ebay.com/2-X-2-X-12-ALUMI...em190035691310
It's 1/4" thick instead of 5mm, but that means it's about 1mm thicker than what you want. From what I see, the only problem is the shipping cost.
Come to think of it, I think I got my 2" x 2" x 1/4" aluminum angle from Online Metals, and it does appear that it's not quite 90 deg. However, I haven't measured, so I'm only going off of looks.
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz