I bought mine from weldequip for something like £18 for 10 though I don't know if that was special price (I'd ordered a lifetime supply of grinding consumables). Struggled to find them elsewhere on the net at a decent price.
The other things really worth having are clean and strip discs - like foam with sandpaper in. Very effective for stripping paint.
If your buying a shed load of discs especially grinding and cutting watch were you store they absorb moisture which will increase the risk of then exploding when used.
As for bulk buying any trade counter will sell you as many as you want the more you buy the cheaper they get.
I just bought a lifetime supply of grinding discs (reason being I keep forgetting to buy the things and end up short) - so don't be telling me they won't last. I'd better keep them near the boiler in sealed packs with silica gel and hope the mice don't get them. (6 mice from that cupboard in the last 24 hours - a personal record. Humane traps of course - mice killed very instantly).
Bibielle DLF115Z flap discs 115mm conical blue zirconium
Fibreglass backing gives excellent strength.
When the backing comes in contact with the work surface it does not damage or cause any smearing. Allows full use of the abrasive cloth.
PLUS - PURCHASE 100 OFF 115MM FLAP DISCS TO RECEIVE A FREE MAKITA 9554NB 115MM ANGLE GRINDER.
Usually despatched within 1 day. Details of our carriage charges.
Klingspor flap discs are worth the extra imo.
A lot of people use them wrong, wearing out the edge before the rest, making them pretty useless.
Try and keep the whloe flat section on the metal, rather than using it like a grinding disc.
I bought some Garryson ones, they're much thicker than any others I've seen so far. Got some zirc ones and ali oxides, seem to last well. I know Weldequip sells abrasives so maybe he could do a sample pack of discs so anyone buying some could find the ones they like most without buying several at a time?