Well, there's two approaches you could take here, LED or regular halogen.
You're going to want to use this website: http://autolumination.com/
(once you get past their atrocious design...they're really a fantastic website)
According to the silvayna bulb replacement guide, the rear turn signal bulb is a 7506 bulb (it's probably labeled on the light block as a P21W).
If you want to take the LED method, go to this page http://autolumination.com/1156_1157.htm
and search the page for "44 High Powered Wide Angle SMT LEDS". It should only have one result on the page, it comes in Red, Amber, and White, and it's a CANBUS LED, so it should not throw any errors or cause problems. HOWEVER, the 06/07 E9x's are very finicky with LED upgrades or HIDs--even if they have error cancelling modules. So it's a gamble, but it could come out really nicely. But at any rate, if you want this ones, pick the 1156 in AMBER
If you want to take the halogen method, but don't want to see the ugly yellow bulb in the housing, try these bulbs: http://autolumination.com/chromedomeseries.htm
Pick the 1156 in AMBER.
Best of luck.
Pick the first bulb on the list