In reply to Happeuss:
Depends on just how fancy a do it is.
Getting shirts that fit all over is a bit of a nightmare.
If it's a very very fancy affair, you might feel a bit underdressed unless you have a wing collar and either studs or 'fake studs' on your shirt. If this is what you're after, then your choice is a bit limited, and you may end up needing to buy and get alterations done. As noted already, darting a shirt is an easy same-day job for a local alterations tailor. Depending where you are, should cost £5-20. Buy a shirt that fits you on the collar, shoulders and sleeve length. The waist can then be brought in. I would, however, advise to avoid this kind of shirt unless you're convinced that this is a very very smart affair - wing collars look awful unless they are the proper detachable sort, and since you certainly won't be getting one of those, it's worth trying to avoid. If this route is necessary for you, any of the major shirt shops like Charles Tyrwhitt, TM Lewin or Hawes and Curtis will have something, as will larger department stores and most hire shops.
If it's only a bit fancy (ie paid ball for fun, not a spendy charity gala or awards ceremony of any sort), then you're much better off in an ordinary white shirt with double cuffs, which is the more traditional choice for black tie. The best bet for these is Charles Tyrwhitt. If you can get to one of their bigger branches, they sell each shirt in a choice of lengths and in three different fits, and they're very cheap. You'll need their super slim. Remember to get a shirt with sleeves long enough to reach the base of your thumb and a 'skirt' that covers your bum. Get something in a plan white fabric with a generous collar, and use the collar tabs to keep it flat when worn. They also sell online, so you can check their size guide, crack out your tape measure and order a well-fitting shirt in time.
You might already be all over this, but you can also improve black tie a lot by buying the bow tie. get a self-tie. Will take 5 mins to learn how to tie it on the web, and looks massively better than a tired rental fake bowtie.
Polish your shoes, keep your jacket fastened when standing, put a white handkerchief in your pocket, and even in a dodgy hire suit you'll look much better than a lot of the folks there.
Post edited at 09:52