Wherever you decided to get your customs from, be sure to be extremely clear when your deadline is - which should probably set a reasonable time before the actual event, like a month ahead if you can. Three months sounds like plenty of time to get anything done, but just ask any of the chaps here who've had to wait longer than that for their addiction to be appeased ;)

FM has a very good point. You're going to be stuck with a handful of stock tartans for the rentals, so you, your father, and your best man will be set quite apart from them, unless you happen to choose the same tartan. Even then yours will probably be better all around since they're going to be customs and not rentals.

I don't think you mentioned where you live? Your locale might have a rental shop you can check out, too.