We used cloth with ds (and also EC'd beginning around 4 months)...
I've changed more disposable diapers than I care to count while working at private foster home and various babysitting stints. I believe the benefits of cloth far outweigh the convenience of disposables. I bought some of our diapers online and some in a store. We tried many different kinds, pockets, flats, AIOs, prefolds, fitteds, etc... In the end I found fitteds and prefolds to work the best for us and both were more economical and held up better than the alternatives.
Washing is easy, we always used BS and vinegar and when weather permits we hang outside to sun and dry, otherwise we often hang from a rack indoors. I think there can be some learning curve at first to find the right washing routine, but once you get the hang of it, it's not a big deal to throw a load of diapers in, and we didn't find them to make a significant difference in our utility bills.
You mentioned that you were interested in cloth to for environmental reasons, but I would encourage you to do some research about the possible health risks of disposables...
http://www.everydaydiapers.com/category_s/26.htm
To answer your specific questions:
Where do you get cloth diapers, is online my only option?
I'm not sure where you live but there are lots of stores that sell cloth...We live about 2 1/2 hours from the nearest store that stocks them, but manage to pick some up while we're in town. We bought a good portion of diapers online.
How much money did you save?
This is hard to estimate. We rarely dry our diapers and washing hasn't caused us any serious change in our utilities. Since a very small amount of your preferred "washing" ingredients are needed it's not very costly. I think a lot of the savings depends on what type of diapers you buy. There are so many types of diapers, but as I said earlier, I found fitteds and Prefolds to be the best value. Practicing EC also cut down significantly on the wash load and how many diapers we needed.
Anyone that used both cloth and regular and found one is easier/better to use than the other?
Having used disposables before (and even breifly with ds before we managed to find the cloth we wanted and have it shipped while learning as much as possible about the different choices)...I seriously prefer cloth. I could rattle off the reasons, but there are so many advantages, from saving money, avoiding the health risks that disposables pose and being more environmentally friendly, to cutting back on consumerism.
Are there any downsides?
I don't think we found any downsides...
How many will I need to have on hand?
I think this really depends on the age of the child, what kind of diapers you choose, etc...
We bought a TON of prefolds and fitteds and about 7 covers...I used to wash every other day but we had many days of being diaper free with EC although I used prefolds to clean up misses.