Aside from models, accessories (sometimes called 'stages') are also in particularly high demand. Many of them are only distributed via the VPVP Wiki, or redistributed by the gracious souls (or simply shown off by the braggarts) on youtube, and other emerging 'English language' MMD sites.
Miku Miku Dance actually comes with a set of 'starter accessories'. Miku's leek, some laser lights, a couple of stages, a microphone and a cute pair of kitty ears. These can be positioned in the 3D space given, or attached to the models themselves to give the appearance of holding or wearing that item.
Aside from the Vocaloid Accessory format (.Vac) items included in the program package, there are also a few accessories in the Directx (.x) format. This format can be outputted by most 3D software titles, and is native to the Windows OS. Just like the .AVI (Audio Video Interleave) video format that MMD exports.
I've compiled here, a near identical list of accessory sites to the one seen on the VPVP Wiki. Here, you'll find many different spots for downloading, though the files are hosted on a variety of different servers. The methods for downloading each file may differ, depending on the system each site has set up. Also see the new Accessory Index.