If it's sims1 I'm impressed, skins for that game rarely have a great level of detail. Being a huge sims2 fan, however, if it's from a later game, the gown may well be inspired by an existing garment. Take a look at the creator's other works, I know I have literally hundreds of outfits based on real world clothing lines sitting in my 22gigs of downloads.

If you've found an image of the ideal dress, that's already one hurdle down! Even if it is from Sims, now you can show it to others or look around the internet or IRL stores with a concrete "This is what I'm looking for," rather than trying to describe it each time. :) Good luck!

Every time I play sims my sims always end up with floating grey centaur bodies. Even the babies. Ah, great memories.

Take an image of it and make it custom...it would be worth it OP

I bet there's a way you could get that or at least, something quite similar.

There are thousands of styles of wedding dresses out there - and you may even find that dress outside of a bridal shop, as a regular party or formal dress. I'm curious what the dress looks like, I would totally love to help you find it!

If your real wedding dress doesn't end up being as nice, just recreate the wedding on Sims and live vicariously through your virtual self.... It's almost as good as the real thing!

Tailor? Art student (fibers or fasion majors)? Commission someone...

solution: have an online marriage

you could custom design one