I worked hard on the Rocky folks back in '00 for Worlds. Guy in charge knew all about disc golf, was at one point a casual golfer himself. No dice with any sponsorship, however. Our numbers are pretty puny so we wouldn't hit the radar for many of the big-wigs out there.
Lowes, Menards, Lumber Companies- for all of the hardware, lumber (tee signs, tee box forms, hole markers/guides) course maintenance tools (rakes, shovels, saws...etc) concrete (if you do it yourself) Grass seed for new fairways.
Also, possibly bulk plastic manufactures- Dow, GE, BASF...etc. Not sure where the disc manufactres get their bulk material from, and we probably couldn't find out that info due to trade secrets, but those companies have buku bucks for advertising and sponsoring. I know you have seen all of the BASF commercials, their slogan "We don't make the products you use, we make them better". I could see them showing a disc golf player hucking a disc off a tee pad in one of their commercials.... "We don't make the disc you throw, we make it better" That'd be sweet!.