You could choose to wear a dress previously worn by someone in your family such as your mother or grandmother. You could also wear a friend's dress.
Another option is to buy a used dress at a consignment shop or through an advertisement in the newspaper. In some cities, there are even consignment shops that specialize in bridal gowns.
Making your own dress, if you are an experienced sewer, can allow you to have a dress that would have been much more expensive if you'd bought it commercially, and will probably fit better and be better made than many commercial gowns. (Two tips: Use a dress form, and be sure to make at least the bodice in an inexpensive fabric to check the fit and construction before making it in the final fabric.)
If your sewing skills aren't up to making the dress yourself, or you
don't have time, hiring someone to sew it for you can still save you
considerable money over buying a commercial dress, and you get the same advantages of better fit and construction.
Commercial dresses can be purchased less expensively through Discount Bridal Service. Other possibilities are to buy a store
sample, or to order at a trunk sale.
Finally, you might choose a dress that isn't marketed as a bridal gown. Many lovely bridesmaids' dresses come in white or ivory, and are considerably less expensive than bridal dresses. You might also find an appropriate dress being sold in ordinary stores. It is also
possible to wear something that doesn't look like a typical wedding
dress, such as a lacy blouse with a long skirt, perhaps in a strong
color such as royal blue or forest green.