Most Nikon SLR's for example use LR-44, SR-44 type 1.5 button cells, these cost anywhere from 50c to 8 bucks each depending on where you buy them and the quality of the battery. You can also use the longer 3V cells but they are often not cost effective.
I do however recommend you ONLY use SR (silver oxide) batteries where possible as the discharge curve of the SR battery is flatter over the rated life of the battery which results in more accurate and consistant light meter readings. LR (magnesium alkaline) batteries have a steeper discharge curve that causes light meter errors as the battery discharges. SR batteries cost more but perform much better.
Many older cameras from the 60s and 70s (eg: Nikon F2AS, Older Pentaxes etc) use a PX-13 or similar mercury cell, a battery that is no longer made due to international restrictions on the production of mercury batteries for environmental reasons. If this is the case then you either have to have the cameras lightmeter re-calibrated, use a Wein Cell (around $10 USD, lasts about 8-12 weeks) or find someone with a supply of mercury batteries (now going for upwards of 10-15 USD each cell).
Other fun batteries include the dreaded PX28A, PX640A, PX675, PX21 and PX28.
What camera and model r u using? If can, get the SR44 which is silver oxide version of LR44. Mustafa sell $0.80 per pc for LR44 and $3 for SR44 (Maxwell brand). I think I normally get LR44 for $2+ for 2 pcs not 100% sure. Basement of Sim Lim Tower is also a good place to get batt.