is why they are so expensive. the most commonly used size is the 48 inch bulbs. where are these fixtures...and why cant you update your fixtures to the ones that use 48inch???? is it the T12 bulb,,if so I found a place doing a Google.com search that has a case of 6 for $32.34....Thats what you will need to do is by them in large quantities to get a better price per bulb cost.
Search Google.com your self for 36 inch flourescent light bulbs and check out the available sites selling them.
good luck LJ