Mark, you can order the bases from any Bosch Service Center but I suggest buying another combo kit. This will give you two complete routers so you don't need to switch bases and has worked out well for me. Check on Ebay and craigslist; these bases are usually available. Note that the newest version of the 1617EVS uses an adjustable round sub base plate which requires a centering cone that is not included in most kits. This new design allows you to use your router in any position relative to your work and know your cuts will be exactly the same.
"Living in the D" (this means Detroit!)
"It's fine to disagree with other members as long as you respect their opinions"