www.juststrings.com
You can make yourself a set from singles. It also might be cheaper doing this thas buying from a brand.
I also think DR does custom sets, I saw a 7 string set on ebay a while ago which were a set of DR Highbeams. The guages were funky so I dont remember them really, I rthink the B string was like a .137. I'm sure if you email DR they'll give you a quote.