ASSA is a great place to start for breeder recommendations. Some people here may also be able to PM you some breeders near your area if you tell where you are. I know around here if you call one person and that person doesn't have puppies they may be able to point you to someone who does. I give my breeder's name to anyone who (seriously) asks. My neighbors called her and she helped them get a great Sheltie from another breeder even though she didn't have any puppies at the time.
Even a reputable breeder in another state/area may be able to help you if it seems there isn't much in your area. Also look for Sheltie clubs or try to go to a breed show. This may be an overgeneralization, but I think all the best breeders know each other and others in the community. We were also willing to drive quite a way, including to another state, for the right puppy. Around here (Illinois) Shelties cost $700–$900. I got quotes from several people and they were in about the same range. (Don't pick a breeder based on saving $200... but it doesn't hurt to get a sense of it.) The one puppy mill type place I (accidentally) contacted was actually much more—like $1,500.
Be prepared to wait. We had to wait 9 months for the right puppy at the right time. It stinks to wait but it is worth it. I also was frustrated that many of the breeders didn't respond or took a long time to respond.... but anyway.... eventually we found someone. I also wasted a lot of time surfing around Sheltie breeder sites—often they link to each other. And I called several breeders to do phone interviews. We went to one breeder in the area that we decided not to go with... she was acting kind of suspiciously on the visit, worried about the neighbors seeing us and refusing to tell how many dogs were in her basement. Follow your gut and be picky. It is a 15-year investment.
I believe SN has some recommendations on the site about what to look for when buying a puppy. My personal soapbox is to pay attention to the parents—often the puppies get their personalities (and looks) from them.
Sheltie parents should be checked for hips, eye (collie eye disorder), and thyroid. The breeder should be able to show and tell you the results.
Good luck!