Ebay - depends what you are buying... it's fairly risky at times (I've bought maybe 4 times and had to go through the complaints process to get what I paid for delivered on one of those occasions).
There's NO contest here, a mono laser is the only sensible choice if you intend to print 100 sheets a week... inkjet costs per printed sheet are poor in comparison, and the speed of the printout is impressive in comparison. Simply.co.uk are good, I've had problems with DABS and others in the past - I buy quite a lot of PC kit every year and I either buy over the counter at PC World or I order from Simply (or their business arm, Misco). My current mono laser, like most of my printers, is now a couple of years old - a Brother HL1440, I buy double capacity toner cartridges for it, they're about £45 a pop and last 5-6000 pages.
I also run :Minolta Magicolour laser for colour laser, inkjets are Epson 950, HP970CXi, and a dinky little CD printer from Teac.... I use the Brother for all the niff naff large prints. Save more by buying your paper in bulk - I tend to use Viking for that sort of thing, HP or Viking 100gsm stuff looking at maybe 3.50 a ream.
Dave