Maybe try Aeropostale or a thrift shop
Can I get advice on decent dress shirts for low cost?
Try a local thrift shop. That is the best place to get affordable outfits.
Just double check the quality before buying, or take along someone experienced in buying from these stores.
Thrift/charity shops are a good starting point, but have you thought of hiring one? If it's something you will literally wear once, there's probably no point in buying one. Shops that hire out suits for weddings etc. Will have them and they should not cost too much if you can return it quickly.
Another thought is eBay. You might find a second hand or even a new one on there at a reasonable cost. Most people buy them and then find they don't have a use for them so there should be a good number of listings.
If you do go down that route it might be worth making sure that the seller accepts returns in case it does not fit. Unless you are confident of the size you require of course.
The plus side is, when you have finished with it, you can relist it and find it another new home - getting your money back in the process... or who knows, you might even be able to turn a profit! :-)