It all depends on how far you want to take it. My cab is designed for 1-2 plants. When I built it I thought I could do it all for $200. Most everything in it is DIY and I have close to $1500 in all of it now and I am still not happy.
As for where to buy lighting and things. I went to local electric supply places to buy my ballast and sockets, I ended up paying as much local as I would have online with shipping. For my nutes I got lucky and found a local garden/pet supply place that carries just about every Fox Farm product and a local nursery that carries Botanicare products. Just shop around your local area and see what your community can offer. The only real things I have had to order were my bulbs.