How to find a Ragdoll cat breeder. Here is one way to approach finding a reputable breeder. This is likely good for any breed of cat, but will address Ragdolls since this is the Ragdoll subreddit.
A cat is only a purebred cat because it meets the standards set by the cat associations. The standards set by the cat associations include things such as physical characteristics and personality, but they also include lineage and whether outcrosses are permitted. Ragoll standards for all clubs/registries currently state: Outcrosses: none. This means that a kitten with one Ragdoll parent is not a Ragdoll. A cat that happens to look like a Ragdoll is not a Ragdoll. A cat from a shelter is not a Ragdoll. Note that these other options are still great pets, but if you are trying to buy a Ragdoll, it's likely that you'd actually like to get a Ragdoll for your money.
The best way to make sure you get an actual Ragdoll (and not a cat that happens to look like a Ragdoll but isn't one) is to find a reputable breeder. A reputable breeder is one that participates in the cat fancy: is a member in good standing of a cat club, participates in cat shows, and has a standing (not just "membership" but actual standing) in at least one cat association--in the US, that would be TICA, CFA, or ACFA. Note that "member of TICA" means nothing if the member has no standing within the organization. TICA does not certify any breeders as fit and even state on their website that it is up to prospective buyers to do their own due diligence on any potential breeder before conducting business.
The best way to find a reputable breeder is to attend cat shows and talk to breeders. Since attending a cat show isn't exactly easy in this year of the pandemic, the next best way is to research past cat show entrants and talk to them. This isn't as hard as it might seem, as TICA and CFA publish lists for your convenience: Here is The Cat Fancier's Association (CFA)'s "Top cat list"
https://cfa.org/ragdoll/ragdoll-top-cats/ and TICA's "list of champions"
https://ticamembers.org/standing/2019/RD/You might be tempted to check out club "member lists" but remember the caveat above, membership in a club is not enough, the breeder should also have standing--proof that they've participated in cat fancy activities. They do not need to have grand champion lines (but wouldn't it be nice?), but when a cat is shown, the pedigree is checked, because a cat can only be shown in a category for which it is eligible. Telltale signs that they're a backyard breeder include no standing in any association (other than "member of" or the false "TICA certified" which does not exist), "rare" colored cats, or any cats that are mink, sepia, solid, or white, as none of these cats would be eligible to be shown, since they are mixed breeds and not Ragdolls, and they do not meet the standards set by the cat clubs.
Remember that your goal is to find a breeder that breeds to breed standards and "to better the breed," which essentially means breeding healthy cats that meet those standards. Every Ragdoll cat standard starts with the assertion that a Ragdoll is a blue eyed, colorpointed cat. Only a colorpointed cat will have blue eyes; only blue eyed cats will be colorpointed. Many reputable breeders prominently display a website badge that says "True Ragdolls Have BLUE Eyes Because they are a Pointed Breed."