First thing you need to establish is what type of doll you want to work with. Your style of art, the character design, and accessibility can all play a role in determining this.
I personally love to work with different doll types since it really opens up a wider range of characters I can create.
For example, if I wanted a more realistic approach, then I go for Barbie. For fantasy and exaggerated looks, then Bratz or Ever After Highs are my go to. Some dolls are easier to work with than others (EAH and MH), so maybe start on those and work your way into smaller faces in the future :D
I get my dolls from thrift stores, Amazon, and local stores. Ebay is also good for bulk buying dolls, but if it's your first time, go for a thrifted doll first :D