Sorry.. I missed the part about 18W.. oops.
You will probably only need a WH2.
Specs..
Maximum Power: 35W
Model Number: WH2-120-L
Maximum Current: .33 AMP
Operating Voltage: 120 VAC
Ballast Case Size: H 1.25" x W 1" x L 5.5"
Ballast Weight: 7 oz.
http://www.fulham.com/Detail_Ballasts.php?ID=WH2-120-L