$500 is cheap for a pedigreed Ragdoll or Persian. Most reputable breeders START at about $600 for pet quality and into the thousands for show quality.
You're not going to find kittens of these breeds for less than that. It costs breeders a lot of money to develop their lines. They spend a small fortune showing their animals, earning titles, getting a name for themselves. Not to mention the vet bills, food, supplies, etc, etc.
You won't find kittens of either of these breeds at shelters but you can occasionally find adult cats. Not so often the Ragdoll but quite often the Persian since they're so high maintenance. Sadly they're often dumped at shelters with terribly neglected, matted coats.
Have you researched what it takes to care for a Persian? They need CONSTANT grooming. They need to be fully combed - NOT brushed - at least every other day ALL YEAR LONG. Ragdolls, on the other hand, have a very cottony undercoat and only require a combing about once a week.
Have you also learned about PKD - Polycystic Kidney Disease? This is an early onset form of kidney disease that Persians are genetically predisposed to. Reputable breeders always test cats before breeding and never breed PKD+ animals. Bad breeders DON'T - which means entire litters of PKD+ kittens. A PKD+ position kitten can appear just fine yet up end in Renal Failure and dying as early as four years old.
Please don't get a Persian unless you truly will dedicate the time it takes to care for one properly.