I trained through CBI - loved it, fantastic course and didn't have single complaint the whole way through. I looked at both CBI and OB beforehand - they are definitely very different courses. I found the content of the OB course to be more "touchy feely" if that makes sense. The CBI course had that but it also had really good nuts and bolts stuff to back it up. Alot of theory on the physiology of normal pregnancy and communication...which is a massive part of being a doula. They each have their strengths but for what I was looking for I found CBI to be great.

Are there any other online study opportunities?
Not that I know of

Which did you choose and why? Is the much of a difference?
Oops already answered that above

How easy/difficult was it for you to get your three births?
Easy. You'll find that there are lots of women out there want a doula but can't afford to pay, or who are interested in the concept enough that they'll have one if it's free but not interested enough to pay for it Either of these groups will go for a trainee.

Are you able to do more than this?
I did two more than I had to because the mums approached me and I wanted the experience. I was very glad I did because the very first two births I did were second babies and they were smoooooth sailing and very quick. But the next two had longer labours and other issues during pregnancy and labour so I got to "practice" a bit more lol.

What are the costs like and do you pay upfront? I've looked at O/B previously but I think I got the wrong information.
I can't remember now what CBI was, but I know I paid upfront but you can pay it off. They just don't let you have the manuals until the full amount is paid. You just work off the student website until then (which is excellent anyway!)

Good luck