Mother's Dress

I called Adrienne's and I never realized that the mother of the bride dress is very much treated like a bridesmaid's dress - you have to go early, order it, wait for it to come in etc. My wedding is 7 weeks away and truthfully, my mother doesn't want to spend a ton of money. Does anyone have any ideas on where to shop with a large selection of more 'mature' styles where she can possibly buy off the rack with a few alterations. She is about a size 16 or 18 so she may be a little larger than the standard try on sizes. Any thoughts are appreciated. We are planning on going out on the 28th. Thanks.

Dress her in the color of you maid-of-honor