Your best battery supplier will be close to home.
We recommend lead acid batteries designated “deep cycle” and intended for marine use. You will need four 12-volt batteries wired in series to supply the 48 volts to run your motor.
In my own boat for the last six years I have been using four 120 amp hour deep cycle marine batteries from a local store. They each cost $75 and weigh 64 lbs. After four years they seem as good as new and have required no maintenance. The key to good battery life is to avoid fast and deep discharges.
Since you will continue to use 12 volts for your ships power, you will need a dc to dc converter, 48 volts to 12 volts. A good online source for these is Kelly Controller:
http://kellycontroller.com/

A note on battery installation:
An auxiliary sailboat will typically have a place for two large batteries. On Teal, these are under the v-berth. You should be able to use these existing battery mounting spaces, wherever they are on your boat, for two of your four batteries. Your challenge will be to find space for two additional batteries. Since you will have lots of freed-up space in your engine compartment, that is the likely place. I have not used battery boxes, but these are widely available and inexpensive if you prefer to use them. The important thing is to provide a really sturdy system for holding your batteries in place. Batteries are heavy. Imagine a knock-down situation, and secure your batteries so that they will remain in place even in that event.