This Extension publication "Got Bugs? Get Bats!" has great information and includes building plans if you have someone handy:
Here too is our Bat page: https://extension.umd.edu/hgic/topics/bats
Check out Bat Conservation International (www.batcon.org) which is a wealth of information: http://www.batcon.org/resources/getting-involved/bat-houses/bat-house-buy
Wildlife/Wild bird stores may carry them locally in your area, but be sure to read the links above so that you get one that is appropriately built for their needed habitat.
Good luck. This is a great thing to do.