This Lighting Wholesaler (Lamp Specs in Surrey) seem to sell just about every type and wattage of lamp that an enthusiast could ever need*, with MBFs starting from under a fiver and SONs starting from under a tenner. The bad news is SOXs start from, ouch, £23.
Prices exclude VAT and postage. I'm afraid I haven't ordered from them, but a quick 'add to basket' exercise reveals you can order individual bulbs. Just beware there is a larger postage charge for bulbs longer than 3 ft, and a larger postage charge for orders under £10 (for example, one 80w mercury will cost you £16.73 including postage and VAT, but two 80w mercuries will cost you £13.86 including postage and VAT).
*with the exception of the 180W SOX bulb
I couldn't find any discharge bulbs for sale on
Harrison Lighting's web site, but they did sell me a 250W mercury bulb a while back, as an accessory to an old lantern I bought from them, so it's worth keeping them in mind. As I recall, their customer service was impeccable and I would highly recommend them!