The boning in a corset gives you more than enough structure to hold those babies up long into the night and there's virtually no need to re-adjust them at any point when wearing one. I've gone through dozens of strapless bras in my many years and I've had the same problem with all of them. And that kind of makes sense since larger breasts + strapless bras = hell. With a regular strapless bra, there's nothing to hold them up to the glorious stature of which they deserve!
But with the boning and restrictiveness of a corset? Everything makes sense and is held in place perfectly.
I'm sure you can find a relatively inexpensive one if you search online, but if you find yourself in New York City - I insist that you stop by the Orchard Corset Center on the Lower East Side. You can literally walk in the store and the woman (or man) working there will take a look at you and give you the perfect fitting corset. It's incredible.