Couple of observations...
Framing supplies like matboard and foamcore are big. Shipping will be a significant part of your expense. In the medium to large sizes such items will usually need to ship freight rather than UPS. Freight shipping is the curse of the small, isolated framer. For instance there is about a $100+ minimum charge for freight shipping. (And then they they skewer your foamcore with a forklift prong). You need to order a lot of stuff at the same time to not get killed by the minimum.
If at all possible you should try to find local suppliers for all such oversized items, let them bear the shipping expense and hassles. I would be willing to occasionally drive a fair distance to avoid freight shippers.
Almost all framing suppliers are very behind-the-times in terms of knowing what they have in stock. If you really need an item, you will have to goad them into walking back to the warehouse to see if the item is really there. UnitedMfrs.com is for people who know what they want and order well in advance. The phone people are mostly just order takers, they can't give you much information. Framingsupplies.com is just a little better and a little more expensive...if I really need something I always call rather than order online. Both those suppliers have low ball prices and you get commensurate service. I use both of them all the time, I just have to understand the limitations. To their credit, they are very much cheaper than the companies with better customer service. United ships from the NY area, framingsupplies ships from Georgia, that can affect shipping costs and time to your location.
A great resource for the new framer is...
http://www.thegrumble.com/one of the nicest groups of people on the net so long as they don't discuss religion or politics. Any conceivable framing or philosophical question will be answered within minutes. Sponsored by UnitedMfrs.com, have to give them credit for that.
PS UnitedMfrs.com has the most hopeless "Search" box of any supplier anywhere. If you need something browse the online catalog, look up what you want in the index. Same for framingsupplies.
PS if you can make it to Vegas this Monday through Wednesday (Jan 26 to to 28) you can go to the WCAF show at the Hilton. All the big framing suppliers will be there, it would be a great way to get some perspective. Matboard, moulding, equipment, the whole works.
http://www.wcafshow.com