Well, then I'd just ask for a third of a yard. The price will be very close to what you'd pay for a fat quarter there and you'll end up with a little more fabric, as well. I read once that Alex Anderson always buys 1/3 rather than a fat.
I'd prefer for a fat quarter to be cut ahead of time rather than on the spot anyway since there might be more chance for a mistake when cutting. (Not to mention that if I were the cutter, I'd prefer not to be watched). But that's just me!
A lot of times, fat quarters are cut from the end of the bolt not from 'fresh, new' bolts - depending on the shop's policy.
lots2do