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I'm like Donna. I go to thrift stores. I wear an 18 dress also. It is a bit of a chalenge finding 18s. Most of the dresses are 10 or 12s. If you visit regularly, you will find something. I have found several formal dresses there. I don't know where you live, I have found Name Brand Clothing is good too. They have a web site but you can only use it to find a store near you.
I do most of my shopping a thrift stores and find plenty of good bargains. Most thrift stores stock formals and they are very reasonably priced. I used to have a gorgeous prom gown that I bought at a Goodwill store for $20.