I would go local, at least first off, to see what is available within your budget. Most jewelers will bring in a selection for you to look at if they don't have on-hand what you're looking for (at least where Iwork we do). You can always do a print out with the specs/pricing from a reputable on-line diamond vendor of a diamond you are interested in to 'help' with negotiations with your local jeweler.
If you're unsure of the setting, I would definitely put your $$ into the diamond and then look for the semi-mount of your dreams. Good Luck!
If you're unsure of the setting, I would definitely put your $$ into the diamond and then look for the semi-mount of your dreams. Good Luck!