Primark is one place: they are cheap and cheerful!
The thrift shop.
Thrift stores are a great place to find a cheap prom dress, however, just keep in mind that the dress will be used and only in limited sizes. Plus, the style may be outdated.
you can probably find one
You could find a cheap prom dress at a Consignment Store or a Resale Store. You could also check out thrift stores. All these stores have inexpensive clothing.
You can find a strapless prom dress for cheap from the Salvation Army, Volunteers of America, Amazon.com, Good Will, and other secondhand and thrift stores. You can also find one a plato's closet.
Tiza.com has a great selection!
No special style or type is needed. Anything tasteful and modest is okay.
It was a formal occasion, so she wore a modest dress.
You can find an affordable wedding dress at the following address: http://www.bestbridalprices.com. If the dress looks cheap, you can buy and apply fake pearls.
Big department stores like Macy's or JC Pennies sell modest prom dresses. There are many other place that sell prom dress but it depends on how much you are willing to pay for it.
try dressurl.com or promgirl.com
There is no designated dress code for Catholics except that it proper and modest.