Pallets, floorboards, whatever you want to use and how raised you want them. Try Freecycle for getting hold of them or builders yard etc. People on my site sem to use all sorts to construct their raised beds. Some are really neat and tidy, others are just soil scooped up. Then fill with soil, compost etc.
I agree that 4' is excellent for reaching the middle without treading on soil, thats how wide mine are ( for this year, I'm new at this too) although I've just read on another thread that for your bigger brassicas 5' is better. ;D