My suggestion would be to talk to your local frame shop(s)...
When I was starting out I explained to them that I wasn't stealing business from them, but needed raw materials to frame my own work. (This was work they weren't going to get anyway...)
I ended up meeting a wonderful woman, who eventually gave me a part time job in the shop while I was shooting freelance... Our friendship continues to this day and she is my source for metal frames and hardware.
Mats I get from an art supplier in Toronto... By buying in bulk, I get a discount, and shipping is virtually nothing.
Find out what kind of deals are available to you in your area as well as online... You never know unless you ask... Gives you something to compare to if nothing else.
Good luck,
joe