A lot of the breed clubs have web sites. Some have a page with the standard for their breed on it. The standard lists what a bird should look like. It also lists what exact things in a bird are disqualifications.
The breed club sites also usually have pictures of good quality birds, so you get an idea of what they should look like. Feathersite usually has links to breed club sites, on the breed pages. Or you can google them. You can also look at breeder's sites, to see what their birds look like. That can help you get a better idea of what you're looking for.
Before the world wide web was created, some of the show breeders were quietly selling eggs to MM. I think that's why I got some really beautiful birds in the past. These days, it's pretty easy for people to sell hatching eggs all over the country, with internet access. I don't know what their quality is like at the moment. I think there's a bit of luck involved.