Just to prove they exist, here are some F18T8 cool whites. Don't look at the price unless you're sitting down with a stiff drink!

17W should be much easier to find, that is possibly the most common 24" tube now. Still likely to be twice the price of a 48" tube of similar quality!

HD staff have no clue what they are talking about. 15W, 17W, or 18W will all run fine on your 18W ballast. The difference is well within the variation in ballast factors between different ballasts anyway, which means that some 15W ballasts will actually drive a tube harder than some 18W ballasts.

Thanks for the points shrubs! Sadly, I have neither the space nor material to merit a 48'' light setup! One additional thought now: what would you take - 15W Cool White T8, or 17W Warm White T8? I understand the differences between WW and CW interms of seedlings (the oft-recited leaning seedling expts) but would the wattage overturn that kind of convention?

thanks again
K

D) none of the above :)

There are very nice compact fluorescents available to fit a 24" space. If you avoid over-priced aquarium stores you can find a nice 55W twin-tube (PL-L form) 22" long for less than $10, giving 4800 lumens. That's a good bit more than you'll get from any three 18W tubes. You'd need a ballast for it though.