I have been looking for the square mirrors and floating candle bowls, and I am trying to get them as cheap as possible. The Home Depot have the 12x12 square mirrors in a box of 6 for $9.99 and Wal-Mart has the bowls for $2.97 for a 8". They also have larger bowls but I can't remember the price on them. Hope this helps!
I think you can find those square and rounded mirrors in Ikea, theyre pretty cheap too. I like this centerpiece, also found some amazing ones at www.ablazecandles.com/candle-centerpieces/. Think they have a discount for wedding candles if you buy in bulk, rose one is my favorite.