The expansion pak that is sometimes bundled with the DSTT is the exact same as the eWin 2-in-1 expansion pak (in fact, it says "eWin" on it).
They are relatively difficult to find nowadays, but I believe zhuzhuchina.com has them (the owner zhuzhuchina is a member of this forum and you can PM him for more information).
The EZFlash 3-in-1 Searinox mentioned works just as well with the DSTT and is more readily available. There are some differences between the eWin and the EZFlash (EZFlash has a crappy rumble built-in and both fast volatile and slow NOR memory, but the eWin has more of the faster volatile memory and is slightly easier to use because it only has one memory type to worry about), but not enough that you should have any serious reservations about one over the other.
If you're concerned that the eWin doesn't have a built-in rumble (you shouldn't be -- rumble sucks), eWin also makes a separate rumble pak which costs around $4 and is better than the one built into the EZFlash 3-in-1.
-Bri