ebay is usually your best bet-unless you’re into decoding japanese sites. but if you’re starting out i’d suggest getting a knock off first because genuine blythe dolls can be really expensive (and valuable!)
search “color changing eye doll” or “basaak” on ebay and you can usually find some decent knockoffs to practice your customization.
etsy is also a great place for buying clothes, eye chips, and hair for rethreading. i got a cheap little doll making kit from hobby lobby to customize mine a little bit, but depending on how far you want to go with it, you might have to spend more.
generally, ebay has been my best bet in the past. hope this helped!