I would guess that groups like Americans for Safe Access and other marijuana advocacy groups can give you a list. Many cities ban medical marijuana dispensaries based upon recent court rulings. Otherwise, its a simple matter to check with the city you are interested in, and ask what their regulations are...
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There are several organizations that can help you with this. Or just check with the city you want to get started in. A recent Cal. supreme court case supported the right of any city to prohibit opening a dispensary. In L.A., you probably have no chance since there are strict limits to the number of dispensaries and there are no available permits. I suspect the waiting list is very long.
Since Marijuana is still illegal under federal law, trying to do this on your own from scratch is a very bad idea. Find an attorney with experience in the field. See the links below.
Please keep in mind that following a recent ruling that allows cities to ban dispensaries completely, many cities are likely to be jumping on that bandwagon. So, by the time you got set up they city could have banned them. Also, keep in mind that even if the city doesn't have a ban, dispensaries continue to be illegal under federal law.