There are numerous doula organizations out there, DONA, CAPPA, ICEA, ProDoula, Birth Boot Camo, toLabor, and many, many more. The cost is a big factor; find out how much the actual training is, what is the couls to certify, membership fees, and so on. Also look up if you need to recertify with them and if so, how often and what is required? Beyond that, the organziation itself should align with your own personal philosophy.
At Birth Beautifully, we do not require you to recertify, nor do we charge membership fees. Once you are certified, you are certified for life!