Have you tried your local home depot or hardware store? The local home depot here carries fixtures in all different sizes starting at 24" for T-8. Be careful tho, 36" T-8 bulbs are not that common and you may not be able to find the bulbs you want. I have a 40 Gallon tank that is 36" wide and have 4 T-8 36" bulbs on it. Home Depot recently started carrying the Philips Daylight Deluxe nulbs in T-8 form and that's what I run on most of my tanks now... except the tank mentioned above. You can buy the 48" T-8 daylight deluxe bulbs 2 for about $6.50, so you can't beat 'em! The 36" bulbs I still have to buy thru a fish store and pay about $11 each!!
If you have to go shorter that 48", then I'd look at the 24" and maybe overlap and couple of them to get the light you need.
Good luck,