Before you start calling around, you need to know very specifically what you are looking for. Do you want an Open level dog (proven or unproven), a successful Pro-novice trialer, a good farm dog, a started dog or a high octane pup who needs a lot of training but has tremendous potential? If you can scrape together the money, a proven Open dog is well worth the money - the dog will take you "all the way" - no need to upgrade to a better dog, the dog will make up for a lot of your faults as a handler, and will teach you more than you could get in a lifetime of clinics and private lessons. Pro-novice or nursery dogs can be an O.K. investment, but oddly enough, although they are cheaper, they are usually a larger risk for the novice who hasn't developed dog sense yet. The risk reason being that for the same money you may be buying a Pro-Novice dog who can barely do the course, or a young raw dog, an old dog heading into retirement, or it may be the seller just has too many dogs. Things to consider when buying a trained dog, not in any particular order:

1. pedigree (if you don't know, ask someone who does. Bloodline knowledge will enable you to look for specific faults that may run in a family) outrun- they're isn't a future for a bad outrunning dog. Buy a dog with a natural outrun and avoid a dog who stops as he's outrunning. power - pick a dog who has an appropriate amount of power for your use. There is no truly accurate way to gauge this in a dog you don't know. I learn more about my the power of my dogs every day, including the dogs I've raised as pups. Power can be a variable trait in a single dog - when you look at a dog he may be more or less powerful overall than what he looks like on that day. Bloodline knowledge can once again be very valuable here. drive - does the dog "take a line" on the drive? (basically same as fetch balancing just heading out on the drive - you want to see a dog that lines out, but is also easy to pull off balance)
2. good flanks - if you are looking at a dog with a lot of polish you'll want to see the dog flank out. If you are looking at a less polished dog, or one that has had a vacation look for a dog who cuts his flanks a little. Avoid dogs who flank too dramatically or who practice "triangle flanks"

A pro-nov dog will be cheaper, $2,000 to $3,000, but training may not be solid enough for the dog to teach you - you will have to teach it, females will be easier to find in this price range. On average, you'll get a better dog at a more reasonable price from handlers who go through fewer dogs (versus dog jockeys). A dog jockey usually won't guarantee health (hips/eyes) and you usually don't get a trial period. If you find a dog you like you can get a video before traveling to see the dog which will help a lot, a lot of people will allow a month trial on a fully trained dog. You may also find an imported dog (there's actually a lot of imports for sale usually), but don't necessarily look at this as a Good Housekeeping seal of approval. The key is to buy from someone you trust who has a good reputation, willing to stand behind the dog they are selling. Sellers can hide a variety of faults in a dog without a novice noticing. Just because a person's handling hides a fault does not mean they are trying to take advantage of you - it just means they know how to read the dog in order to make him look good. As a novice however, that is where you can get in trouble. A lot of making a dog look good is timing and stock sense-- skills that you may not have developed enough to notice the subtle fault avoiders that a handler would use. As you mature as a handler, you will be able to recognize more things in a dog. The biggest question in your decision should be what kind of handler you are, or think you are. Are you a control freak? Do you panic when sheep are coming fast? If a dog doesn't automatically hit the deck do you get upset or worried? When things are going right, do you stop your dog - just to make sure you still can? If you answered yes, a pushy dog may be too much for you to handle. In order to be successful with a pushy dog you have to be willing to go with the flow and not panic when a dog moves fast. This type of dog is usually more difficult for the novice to handle, but the type of dog that has the potential to be on the top of the heap. On the other hand, do you like a dog that gives you time to think, a little slower? Always lies down - doesn't push? Usually an easier type for a novice. You need to find a trained dog who fits your current style and this will change the more experience you get.

Just a note: you won't be able to run your open dog in novice, but you can run in pro-nov, and it's a wise idea to take the opportunity to run pro-nov before jumping off the deep end in open. It will build your confidence tremendously, and at least 30% of running open is just a mind game.