By far your best bet is to google variations of "Adobe Premiere Elements plugins". I'm sure you'll find plenty. However, be warned that anything you find that might be useful will likely have a $$££ value attached.
Are you looking for anythign in particular? OIr are you just trying to build your library?
Be warned about installing extra transitions. You just might be tempted to use them - which could be a mistake. Unless it's for some special effect then I would be surprised if you needed any transitions other than a straight cut and a cross fade anyways.