Since 18w LPS ballasts are hard to come by and are usually pretty expensive when they are for sale, here's a compilation of alternative ballasts people have used to run them with:
Electronic F20T12/CFL ballast, a lot of people have had success running 18W LPS on these. Electronic plug-in CFL (not the one from a screw in lamp) ballasts have also been pretty good. Most people on LG usually run them using the Fullham Workhorse 2 ballast which slightly underdrives them though.
F32T8 ballast, most newer ballasts can auto sense the lamp and adjust the current automatically. Depending on the age and type of ballast and this can either run them perferctly, overdrive or underdrive. Experiment with different ballasts to find the best type.
240V F20T12 choke, a pretty good choice for a 18w LPS but you'll need a 120v to 240v transformer to use them. Also 240v chokes are pretty hard to find in North America. You'll also need a S-10 or FS-4 type starter in parallel with the lamp to properly ignite it.
120v F20T12 choke, I personally never got this to work with either a BBQ ignitor or a starter. Different chokes may produce different results though. Others have gotten this to work.
F20T12 trigger start ballast, I recall seeing a pic of one running with this setup on LG. Will update with more details.
L69 LPS ballast, the recommended way to run a 18w LPS, hard to find and often costs more than $100 though. Is actually a high reactance autotransformer ballast with enough OCV to auto ignite the LPS.
