Mobile Fidelity Sound Lab pressings. They don't have the surface noise because they use a higher grade vinyl and have better quality control. Since you are demoing a MoFi table you might ask them for some of their re-mastered vinyl recordings to demo with!
I would not necessarily recommend Pink Floyd DSOTM to demo with. The heartbeat sound on the first track can be too intense and cause lots of problems with feedback to the TT. It's fine for normal listening but you don't want it to turn a customer off to vinyl. If your customer is into vinyl then let them play it if they bring it of course.
There are some recordings I'll suggest that are not at all mainstream, and they might not be anyone's cup of tea for music an general; but they have some incredible 'wow' factor when played on a good TT/cartridge.
Star Wars - The Empire Strikes Back Soundtrack
Cabaret - Original Soundtrack 1972
Fleetwood Mac - Tusk - Particularly Side 4
Blondie - The Best of Blondie
Earl Klugh - Finger Paintings (MFSL)
Crosby Stills & Nash - CSN
Van Morrison - Moondance
Pink Floyd - Meddle
Frank Zappa - We're only in it for the Money
Police - Outlandos D'Amour
Good record cleaner - Discwasher D4, D4 fluid (don't ever use VinylStyl it leaves residue). I use two, one for initial cleaning of dirty record/wet, and a second one to finish and help pick up the last bit of dust as it finishes drying.
I only suggest vintage pressings. The new ones have quality issues except any MoFi releases. Diskeeper ultimate audiophile sleeves are a must to keep the recordings clean. You might need to play/clean a record with surface noise several times to get rid of any dust in the grooves, don't ruin that by putting them back into paper sleeves.
Discogs.com is a good place to look for vinyl; it's a bit more expensive but you can probably track down NM recordings more easily with so many vendors. If I buy via discogs I usually make a few picks from the seller and make a package offer if they are open to negotiation, and it helps reduce shipping cost per-record.
If you want me to look for anything here there are several stores with lots of inventory, PM me a list.