I have purchased a couple from Land's End. If you sign up for their e-mail notices they have sales going on all the time, or at least coupons. Just this weekend they had a 25% off on swimsuits. I need to get one myself but I think I missed that sale. they put these "sales" on all the time thru e-mail. My next one will be from them.
I have also found suits (I am a big girl too) at J.C. Penney. I try to look for the better quality suits, I have had too many "cheap" ones that don't last of stretch out really quickly. Macy's is another good place.