I've never done it, but it can be done!
The bridesmaids can get dresses off the rack instead of ordering them through a shop, in fact Prom season is just starting so there should be a ton of dresses to choose from. Tux rentals don't need much notice, or the guys can just wear suits (spring suits are on sale if someone needs to buy one). If the bride doesn't have here dress yet, either, many bridal stores have clearance racks. Also, again with prom season in full swing, I've seen white and off white ball gowns that can easily be wedding dresses, I saw several at JC Penney's (also a good source for the bridesmaid dresses). For a veil, I bought mine off of ebay (a good source for tons of wedding stuff) for $20 and it looked like one for sale at a bridal shop for $100.
If there is going to be a reception, you can have the food catered from a restaurant and have it delivered to the reception site (church hall, rented space, someones home). Call around to some of the bride's favorite restaurants, see if they cater. If needed, you can also rent tents, tables, chairs, glassware/plates/silverware, tablecloths and napkins, anything you ca think of! It's short notice but call around and see what you can come up with.
With 30 to 40 people you can also just have the reception AT the restaurant. I live in an area with about 20 millions restaurants. One of my fav's is a place called Wildfire, it's part of the Lettuce Entertain You chain. They have private rooms you can reserve, and they have several packages to choose from, some very inexpensive and the food is just fantastic (good service too!). I'm sure there must be a restaurant by you that has something similar.
As for flowers, there are several growers that will ship wedding flowers straight to the bride, she just has to put them all together in bouquets and crosaseges and centerpieces herself. You can have a "flower party" a day or two before and have all the bridesmaids get together to help her. Another option is if you have a grocery store with a floral department, go in and talk to the florist there, they can make the centerpieces and crosages, and you can buy the flowers for the bouquets in bulk and make the up yourself. Costco, if you or someone you know is a member, can do bridal packages that include a ton of stuff and the prices aren't bad, you can check it out online at
www.costco.com
Whew. That's all I can think of off the top of my head. Good luck, and if you have any questions, feel free to PM me. I took a year to plan my wedding, but I found a ton of stuff while I reserched and would be happy to pass on any info I have!