Hello! First off thank you for asking us here. My name is Humberto Valle, I'm the founder of Unthink Digital Marketing (www.Unthink.me) although we have some full stack programmers in our team, we don't offer software development as a service, however we do have a partner company that we refer work to. Below are some of our reasoning as to why we trust this company that maybe can help you decide on who to hire as you do your own search.

BetaBulls.com is an industry leader when it comes to full stack engineering. After talking with them we found out that they have done some custom platforms for government compliance systems, some IoT for healthcare facilities and some GPS based project management tool for a large construction company.
As you look around, ask for transparency from the team, they shouldn't have a problem in sharing previous work or references.

As far as BetaBulls.com, their pricing was very flexible. I know they've taken structures of monthly payments with or w/o equity depending on the startup and platform plus they offer a 30 day free trial and or money back guarantee.

If you are a software developer (i'm a novice) you would see right off the bat that companies that offer free trial periods is most likely because their customer service and their work is above average. We like to partner with companies that match our ideals and mission and are seeking for ways to help the startup guys and small business entrepreneurs who need more flexibility. As you search, look for a team that be a partner not a hired gun that is just sitting there waiting to be babysat. Hire those who can be teammates even if they work for another company or for themselves.