There are 124 bundles for TextMate where only 36 are included by default. Until recently, the way to get more bundles has been to install subversion and then do a checkout of the bundles needed (from the shell.)
Now there is a much easier way: the GetBundle bundle by Sebastian Gräßl. All you need to do is download and double click it to get an “Install Bundle” command which you can invoke from inside TextMate (hint: use ⌃⌘T and enter “install”.)
There is also an “Update all Bundles” command which you can use to update your custom installed bundles to the latest version. A future version of the bundle is likely to offer a launchd Daemon that you can install to have updating taken care of automatically.
All bundles installed via the GetBundle end up in
~/Library/Application Support/TextMate/Pristine Copy/Bundles.
For troubleshooting you can use the TextMate mailing list.