Last week, Simon Crompton had a piece in the more superior British GQ on the importance of following the rules of black tie accessories, particularly in what goes around your waist:
One of the problems is that it can be hard to find a good waist covering. Most places will sell cummerbunds (I recommend Drake’s – make sure you pick the material, grosgrain or satin, that matches your lapels) but it is hard to find waistcoats. A bespoke tailor is, of course, the best option as a waistcoat is one of the hardest things to fit: my personal choices would be Henry Poole or Anderson & Sheppard. Retail, the best resources are probably Ralph Lauren Purple Label or Tom Ford. Both produce separate waistcoats with fantastic detailing and both will do made to measure if you like the style but want a better fit.
It is unfortunate that is difficult to find the proper low cut waistcoat, and I agree that finding a custom tailor to make one for you is your best bet. At retail, Brooks Brothers offers some options as well.
As for cummerbunds, Drake’s does sell a pretty nice one, but the maker is unimportant as long as you find one in a proper fabric that matches the lapels and bow tie. Make sure when wearing it that the pleats also face upwards.