18" T5 bulbs are not standard, with Currentbeingt the only manufacturer that I know of. Their bulbs are junk IMO
A 24" fixture as mentioned above would get you into 24" bulbs, which makes ATI, UVL, GE, Geisemann, and KZ available. All of them put ou significantly more light than Current bulbs. In the long run, will work out better for you if you stick to standard bulb lengths (12, 24, 36, and 48 inches)