You can do it in many different ways. The key is finding the right resource, professional, responsive, reliable and trustworthy. You can use freelance sites like odesk, upwork, etc. but it requires management and sometimes you have to start over.
I personally developed a model where startups I invest in get access to the experienced tech resources (my team). This way startups just need to worry about business side of things until they have enough funds to build a dedicated team.
I think this is the best model as you get access to the team that vested in your project and can scale up and down depending on the need.