It's never too early (start now!) and it's also never too late to try.
When I was trying to find my own doula, I read a book about doulas and figured out what was important to me (examples to think about: certification, number of births, personality "fit," experience with natural birth, etc.), and came up with a brief list of questions. Then I tried to come up with a list of all the doulas in the area -- asking my midwife, asking friends, chasing down "leads" ("I heard there's a doula associated with this store downtown, call them") googling "doula" and the name of my city, etc. Then I started doing brief phone interviews to narrow things down, and knew when I talked with my doula that she was "the one." We met in person just to be sure, but I already knew.
(By the way, I thought certification was very important to me when I started, but it ended up not to be important to me at all, my doula had been to more than 100 births, but she actually used my birth story as part of her DONA certification. The number of births she'd attended, her former life as a Bradley Method instructor, and her personality fit ended up as the keys for me. So it's helpful to stay open on your criteria.)