I agree with Heidi on both counts. Plan your tank for the future. Also, there are some very good online retailers. Live Aquaria is a staple, but i've also bought some of my coolest and most unique pieces online. Pacific East Aquaculture is great to deal with too. I'm sure there are others too.
Also agree that if you can find it locally from someone one in a forum (or one of their sponsors) then that is usually your best and cheapest bet.