It will depend on how much you want to spend, and how quickly you need them......
..... SJ Sportscars list them under part number SJ100J0022 at £15.95 each + P&P + VAT (2 required if both callipers need doing).....
..... but for a much more acceptable price of £4.32 they are available from
rock auto - but will be posted from USA (£9.13 P&P) - hence £17.77 for 2....
.... the part number you need is Wagner #F117251- if you use the drill-down menu and look under Pontiac - 1991 - Grand Prix - 3.1L V6 - Brake/Wheel hub - Calliper Repair Kit (or just search on the part number!).
There are other makes listed, and I'm sure they could be found cheaper at other suppliers, but they are the same callipers as Lotus fitted.
It normally takes 5 or 6 working days for a Rock Auto order to arrive - but at less than half the price, it's your choice!
(if it was me, I'd add in a CARLSON Part # H5587 to the order (rubber seals for sliders, and brake pad to piston metal clips) - under £3 for both callipers, and make the re-build a lot tidier!
but then you may want to get a set of pads as well (much larger choice in USA than UK - prices from about £4 a set!).... and on, and on, and on......