Are there shaft adapters for DC motors usually used in toys so that I can use rods that are larger than motor shafts?
Whether you are looking for adaptors or couplers, when it comes to small sized parts in this area, you are likely to find that you will be spending a lot of money for one basic part. For instance, I just did some googling around for you for "miniature shaft couplers" and found more than a few suppliers, but all were either "call for a quote" (ie - meaning you can't afford what they want - or they want to sell you 1000 pieces), or for single units were around $25-35.00 USD.
A couple of things to look into would be shaft couplers for toys like Erector Set, Meccano, and VEX robotics - there is probably something in those you could use. You could also try to fabricate your own; you just need a small drill press, some thick-walled aluminium tubing, and a small tap/die set (plus small enough set screws). It isn't as hard as you think (but it does take patience and time).
You could also look and see what the R/C hobby world has to offer - plenty of parts and such there; maybe there are some shaft couplers or such you could use?
Another way you could adapt a motor shaft would be to take the motor shaft, measure it, and use a die to thread it; then take your larger shaft, drill it out, and take a tap to fit the threading you created on your shaft, then tap out the threads on the end of the larger shaft. Once you have that done, apply a bit of thread lock, and thread the motor shaft and the larger shaft together.