We do a lot of work in custom mobile app and website development and also salesforce development.
I would start by looking at the apps and software you like to use that was created by smaller firms (not the big companies). Many of these firms also do client work, which you can find out about on their web site. If you enjoy the apps they produce and they have the expertise you are looking for, it can be a great fit. A good example would be a company like Black Pixel.