Hi Friends,
Having a DRZ 400, staying 9 month a year unused, I have the same problem and looked after the same battery last summer.
Again, Red baron was very usefull and even if they don't have it in stock, they order it from Sing and/or/or Japan, and I got one.
Red Baron is Your serious solution !
On an other hand, I managed the battery problem on long time unused bikes by acquiring an Optimate battery charger which is doing a cycle of charge/discharge....Please have a look at the web site where it is well explained
http://www.optimate-sp.com/ . I equiped the bikes with the relevant plugs and I only have to ask the maid to plug the charger once a month when we are not in LOL. It works perfectly and the batteries of all the bikes we have (Yamaha Nouvo & Fino, Ducati Monster and Suzi 400) are at top conditions when we arrive in LOL.
I think it's good investment which respect the battery which should last longer with this equipement. Otherwith, you'll have to spend hell of a money for this tiny DRZ battery.
A part from this, I thing if you have time for a one day search in Bangkok, you'll find a solution in Klongtom the Chinese mechanic corner in Bangkok. By searching, you find everything for car, bikes and others, there, issue is you have to spend a day, at least, there.
Good luck !