You can buy it in the garden centre - I don't really know of any recognised brand names. Using a mix of top soil and compost will help with fertility. You can also get topsoil on Freecycle from people having building work done - it can be a bit more of a risk, however, as you don't know what's been growing in it!
I bought a mixture of manure, organic compost, and cheap grow bags. The best deals, in multi purchases, were from independent nurseries, but it is still quite an expensive way of doing things. I intend to also top dress with organic compost, as time goes by. I'd also be wary of top soil from an unknown source. You could be introducing all sorts of weeds.