My raised beds were just put on grass and levelled a bit to make them even. I then lined them with thick cardboard, I have some already, but I saw somewhere else on threads that bike shops are good for boxes. This year I am going to line sides with polythene so have been saving all my compost bags for this (
thought need to get a staple gun) I did not do this with the others. I do understand about soil creeping out but it doesn't matter to me and in fact doesn't seem to happen so if you were worried you could spend money on their raised bed liners, or use landscape fabric to go across the bottom and up the sides. to stop leakage. 
Saw that on thread on here as well
Just sowed loads of flower seeds, I have a feeling that my electric propagator is not working properly
It is quite old