And to answer the original questions:
When do you get em?
When don't you get them?
How much are you willing to pay?
Well there is actually a formula to determine this. Let C represent the value assigned to their cup size. If they are an A cup, C is .5, if they are a B cup, .75, if they are a C cup, 1, and if they are a D cup, 1.25. You then give their face, breasts, and behind an individual score out of 10, and sum all three, giving you the value B. B multiplied by C gives you R. Then multiply one tenth of their weight in lbs, W1, by their waist size, W2, and that will be your S value. Divide R by S, multiply by 150, and you have your hourly rate in American Dollars.
What services do you require as a minimum?
If using the formula, you are owed erotic foreplay, vaginal intercourse in your position of choice, and a nice massage. This is, of course, more of a case-by-case issue, though.
Do you ever travel to far away lands to access those of a genetic makeup that you find more attractive/cheap?
If it's worth more to you than the cost of the trip, then yes.