Truthfully, unless you're really lucky and walk into a store at the right time and find the dress on an amazing sale, your best bet is the internet. Now although e-bay has some beautiful wedding dresses for only $50, it may not be the most reliable source.
Your safest bet is somewhere like
and you can get something lovely for under $200 THE SAME DRESS as you would pay $1500 for in a store.
I bought a dress on sale in a store for $500 dollars and nobody notices that it was stained on the back where someone had pricked their finger, plus it was very big on me. By the time it was resized and cleaned it would end up costing me over $1000, so I had to pay full price to get a new one brought in. Be very careful if you find something on sale, and even more careful still about 'add-on' costs. The may want to charge you $400 dollars for an alteration and up to $300 if any cleaning needs to be done.
The nice thing about this website is that everything is BRAND NEW, never tried on in a store, and they are accountable for the quality of the dress.