I'm always sort of stunned when someone is buying a longarm and going to immediately start quilting for others in exchange for money. I'd heed feline fanatic's advise and be very transparent about your skill level with the longarm, or work on your own quilts until your level of knowledge with your machine is fairly strong before you start offering to quilt for others. I plan on getting a longarm at some point, but I can't see myself being ready to quilt for others for a long, long time after that. I'd be so scared of messing up someone's hard work!
That said, I don't belong to a guild (not for any real reason, there are a few in my immediate area I've looked into), and I don't use Facebook very much. I do have my quilts longarmed, when I have the money, and I found my quilter via a Google search. I don't have any quilty friends, and most of the ladies at my local fabric store quilted their own quilts, so I didn't have anyone to ask. I ended up selecting her because she was reasonably close to my house, her prices were in-line with other quilters in my area (there are a few that are waaaaay higher than everyone else. Almost all of the others I've found are right around the same price.) and she had an online portfolio (of sorts), so I could see pictures of her work. She usually has about a 4 month long queue, so she seems fairly busy to me.