If you are thinking of hiring a doula, you should begin searching for one as soon as possible.
There is nothing better than going with the trusted word of a friend or someone local who had a positive experience, but there are also doula services available online that will allow you to narrow down your search for a doula by location, the type of doula you are looking for (birth doula or postpartum doula), their certifications, and the services they may offer.
Certain sites to consider looking at are: Childbirth and Postpartum Professional Association (CAPPA), DONA International, International Childbirth Education Association (ICEA), and National Black Doulas Association (NBDA.)