If you're sure you can get the business to support it, then go to the bank and get a loan. If you aren't able to do that, you're probably stuck slowly bootstrapping. (You're asking for advice, but it's hard to really help because you haven't given a lot of details.) If you have 20 customers after years and it's not profitable enough to hire more developers, I'm guessing you'll have a hard time finding investors. But I don't know. In all cases, I wish you luck and hope you find what it takes to get lift-off. I know how hard it is.

Edit: You might want to take a look at http://www.cofounderslab.com