For a long time, I bought all of my framing supplies (mat board, foam core for mounting, frame sections) from Dick Blick. But though I still often buy my paper supplies from either them or any local art store, I now prefer Art City Frame’s custom cut metal frames for better prices, selection and availability.
I get my glass from a local hardware store that will custom cut any size for a very reasonable price. If you’re in the neighborhood, it’s Cooper Hardware at the corner of 14th and Oak St. NW. I’ve experimented with ordering plexiglass online so I could cut it myself, but it’s a pain and scratches easily. For all of the “non-glare” options out there, I still think regular old glass looks the best and most professional.
I also recommend browsing your local thrift store for quality frame materials. Look for frames that have real glass and frame sections (usually aluminum) that can be taken apart with screws. Though some cheaper frames that use flat springs to hold the artwork in place can work just as well. This is an especially good way to get poster sized frame materials that are quite expensive to order or buy at a frame shop.
And if you’re really serious about doing your own framing, it’s time to invest in a professional mat cutter. You can get pre-cut mats for most standard print sizes, but if you’re going to be framing on a regular basis, you can get a mat cutter for around $100 that is very easy to use and pays for itself with a few uses.
I have a Logan Model 450. The key features are bevel and straight blade cutters that fit into a rail, a “production stop” that makes sure you don’t cut too far and ruin your window, and a parallel mat guide that makes it very easy to set your depth. I can’t believe all of those years I wasted with a hand-held cutter and a frigging ruler, making all of my mistakes on that last cut.