If you get the timing right you should be able to get all the compost you need free from the county. That's what I used when starting my garden, which consists of 8, 8x4 raised beds - similar in square footage to your community plot. You can subscribe to email updates so you know when the compost is available. At the site nearest our house, it vanishes very quickly, even if the weather is rainy. We don't have access to a pickup, but I have numerous 5-gallon buckets (mainly from cat litter) and I put a tarp in the back of our minivan, loading all the buckets into it. I empty them and go back to get more. You could locate all the compost sites that are convenient to the garden location so that as soon as one of those gets compost, you can begin.