EBay is usualy the cheapest source when in bulk, but at that price you could get a selection from PolyMax
http://www.polymax.co.uk/o-rings/rubber-nbr-oring/o-rings-bs001-bs050-nbr
Get any mix of o-rings with a minimum purchase of £10 (ex VAT I think so £12 realy), but then add on postage - probably under £4
It means you will end up paying more than the RAP4 100 o-rings on EBay (£15 oe £16) but can have any mix of o-rings.
You could also get away with a cheap o-ring set from your local DIY shop, this will do for cylinders, but the rest may not be up to the specs for marker o-rings