Concur with the above that just buying "top soil" doesn't tell you where it came from. Accept the fact that you won't have that rich, loamy soil that we gardeners crave for a few years. It will have to be built up over time. Find a local source for your soil. A local nursery that you know and trust should have soil for you. Until you build up a good supply of your own compost you will have to buy it. A bag of compost is either 1 or 1.5 cubic feet and runs from $3.50 to $4.99 per bag. To give you an idea on how many bags you will need the 1 cubic foot bag covers about 18" X 30" X 3". Add about 30% more coverage for the 1.5 cubic foot bag. If you have a truck (or friend with one) you can buy it by the cubic yard much cheaper than several bags. I have a traditional in-ground garden with an 8' deer fence (yeah, we have a huge deer problem here in the burbs because we have too many Bambi lovers) so others here can provide guidance on building the raised bed.
Dan - "Welcome back my friends to the show that never ends..." Emerson, Lake and Palmer - Karnevil #9