Lady Twilight's Great Ponies (mentioned above) is an excellent place for baby accessories, stickers, brochures ID. She's working on other areas too like Ponywear and barrettes which is going to be really useful.
I ususally double check Dream Valley's site as there tends to be incomplete or inaccurate information on occassion, but it's still very good: http://dreamvalley-mlp.com/
Just no that there's no site that's perfect so it's good to have a few of them to refer too. Your best bet is to find a MIP item in order to get all the accessories and colors straight. Search through ebay. There's an auction on ebay for a MIP birthday set. It has pretty good pictures and description, so you can probably figure out from that what you're missing too.
according to the back of the Party Set box, it says it includes:
5 party hats
4 hair ribbons
Cake w/Candles (6 slices)
Pin the Tail game (w/2 different pony tails)
Banner (It's Party Time" folded card w/3 party hats)
Greeting Card (It's Party Time" w/Best Wishes)
Party Panties w/box
Blanket ("It's a Party")
Scented Body Sticker (Best Wishes as a pony, not pegasus. Some packs came with other pony stickers like Hippity Hop)
Information received from picture of the packaging from auction w/the set on ebay. The information in the parenthesis is additional descriptions of the items as know them to be.