There are 3 popular aftermarket tanks for the '96 and newer DR650SE. Clarke makes a 15 Litre (3.7 gallon) tank which hardly seems worth the hassle and cost of replacing the OEM tank because it's barely bigger. IMS is another well know manufacturer and they offer a 19 Litre (4.9 gallon) tank -- this seems to be the most common choice among DR owners looking for more fuel range. The IMS tank offers an increase of about 6 Litres (1 1/2 gallons) so it's a nice upgrade in size. The last option is a 33 Litre (6.6 gallon) tank from Safari, but it's almost too big unless you're planning to ride across Africa or some other trans-continental trip that might involve long stretches between gas stations. Plus the extra weight of that much fuel will impact how the bike handles, and it's also quite a bit more expensive to purchase: the Safari tank costs as much as two IMS tanks, or three Clarke tanks).
I went with the popular IMS tank for $249 from ProCycle (highly recommended) and that included a new gas cap (non-locking) plus a new petcock valve (not the best quality, but it will do for now).
IMS makes this tank in a bunch of colours to match Suzuki's paint schemes. I wanted to choose blue, but I saw a few photos of the white tank mounted on blue DR's and I liked the look. I've already got white side panels and a white headlight shroud. Plus, white goes with anything.
When I placed my order for the tank, I also threw a few other goodies into my online shopping cart: a magnetic oil drain bolt, a pair of spark plugs, and 10 Hi-Flo Filtro oil filters. The entire order came to $325 USD and that included shipping (fortunately the Canadian dollar is trading at very close to par these days).