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.
Take a look at UpWork - there are tonns of developers to choose from.
You can take a look at the Eastern Europe' software development market (for example, Belarus).
1. American Gartner experts recommend to outsource software development in Belarus
2. American Business Persons Say It is a Good Idea to Outsource Software Development in Belarus
3. Working hours in the USA overlap with those in Belarus, but not in India or China