Ask your mom!
My mother had her dress beautifully preserved, and although it's been 40 years, the dress looks new! She saved it, thinking that her daughter might one day have a need for it.
And, it turns out, it fits me like it was made for me! I may have it altered, but that shouldn't cost nearly as much as buying new, but I don't have to.
If you aren't the same size as your mom was when she got married, ask around; maybe you have another relative who would be honored to have you wear her dress!