This would be a good place to start; the Scottish Welsh Pony & Cob Association. It's probably the best way to get to know the cobs and their breeders in your area, and if you go to their shows you'll get to see the stock in action too.
http://www.swpca.org/index.htm
You'll probably find a lot of the foals from the bigger studs are not handled much (if at all) until weaning. Most studs welcome visitors to view foals before then, which gives you a chance to see them at liberty with their mums. Look for a foal that's curious, and wants to stand and see what's going on even if you can't actually stroke them. Look for one whose mum is happy to be approached and enjoys being with people.
Conformation wise, look for a good front with a well laid back shoulder and a foal that moves freely. Some cobs have an impressive knee action but not much else - a good length of stride when they move is much better. Tail shouldn't be too low, and the quarters should be powerful and fairly level. Head shouldn't be plain - but very pony-like heads are frowned upon. (The pics of Blod on the dark bay or black thread are a good example of what to look for - she is actually a very nice example, although lots of foals are chunkier at that age - which is great
) Some lines mature very slowly - Blod is from Fronarth Victor lines who do lay bone down later and chunk out significantly. Others will look heavy to start with, and kind of grow into themselves. Cannon bones should be short, bone should be dense and flat.
Some lines are ideal for driving, while others are more riding type cobs. The ridden cobs should not be light of bone; but they do tend to be longer in the back and less upright in front (if you look at some of the cobs that do well in-hand you'll see they have a very upright front, huge movement and are sometimes quite short in the back, while ridden cobs are better suited to working in an outline under saddle) so if you plan to ride rather than drive (or as well as) look for foals that have ridden cobs in their pedigree.
There is no real 'corrrect' type; this is Teleri, who is chunkier and more compact than Blod, but neither is more typey or correct than the other.
Blod would probably make a better prospect for work like dressage; Teleri would do workers and probably drive nicely. How well either would do in the show ring would depend on what types of cob the judges like. As long as they both conform to the standard (which is really open to interpretation) it's impossible to say which is the better example.
As for bloodlines... it's difficult to avoid Nebo Black Magic in cob breeding, but if you buy a filly I'd try and look for a pedigree where he doesn't crop up too many times (it's not unheard of for him to be registered as five out of eight great grandparents, for example...) just in case you want to breed later. No reflection of NBM as a sire; but it does make it easier to find a sire that's not too closely bred later on. Nebo usually breed cobs with good movement and lots of front and plenty of bone. Fronarth lines give some beautiful cobs; they're quality cobs that tend to be good under saddle. (Ok, I'm biased - all three of my cobs are Fronarth lines...) There's a mare called Trofarth Ebony who's qualified for Olympia this year - I'm sure she's Fronarth lines too.
Try looking at this site - it shows a lot of the most famous cobs (mares, stallions and geldings) and gives you a chance to study what breeding crops up in what cobs too.
http://homepages.manx.net/welsh-cobs/Default.htm
