take a look at the CSA farms listed here on Local Harvest. Just click on the CSA tab in the menu above to get yourself started.
The average cost is around $25 to $30 a week which in most cases you will pay in full up front when you join. be sure you are joining a real CSA run by a farmer or farmers and not a Fake or conglomerate CSA which is really just a buying club that resells products, some of which are from local farmers but most things are not. A great way to be able to tell the difference is ask for a tour of the CSA farm, if they won't give you a short tour that is a red flag. Also go for experience, if you find a cheap CSA but they have been doing market farming less than 5 years you will get what you pay for (usually less than you expected).
You need to join one soon, as most CSA's sell out by mid April