Seeing as most of the patterns stop at 48. and I have a 50 inch bust
I TOTALLY feel your pain. I'm a fairly new knitter, and nothing is ever in my size either. I've devised a couple of pretty successful ways to modify patterns or create altogether new ones inspired by the ones in the books.
My favorite thing is to take an article of clothing I already have that fits well, that way I know for sure it will look good on me, lay it out flat on the bed and measure the width in 1 inch increments going all the way down. You can do some math to figure out "Well, if the gauge is 4 stitches per inch, and I need a 50 inch bust, then I should have 200 stitches total on the bustline, so 100 per side." You can use the general shaping from the pattern and replace it with your numbers taken from your clothing item.
You can also get a scrap shirt, some 50 cent thrift store throw-away that you don't mind wasting, and rip all the seams out. When you see the peices laid out separately, its actually easy to invent your own pattern by measuring that and looking at the shapes.