Here's another vote for jos. a banks.
You'll probably want to buy it somewhere not online because it'll need to be tailored to fit well, and it'll be easier to let the place from whence you buy it do the tailoring.
Also of note: the traditional shirt with a tux has a fold down collar. The tip collar that shows the band of the bowtie (i.e. tips, the one everybody else will have) is for a white tie and tails. I suppose it's common enough to get the wrong shirt with your tux that you can more than get away with it, but traditionally, fold-down collar = black bowtie, and IMHO it looks better.
IMHO again clip-on bowties aren't the greatest thing in the world. Get a real bowtie; ask the guy from whence you buy it how to tie it. It's easy. You'll thank me when you get compliments from attractive girls