So, I am assuming that the midwife that you are working with does not have an account with a lab for testing.
When I was an apprentice, the midwives that I worked with did not believe that homebirth midwives should be doing lab testing. They believed that it crossed the line and was practicing medicine. So, when we had a mother who wanted GBS or other testing, we sent her to one of a few family doctors whom we had developed relationships with and then had them fax the results back to us or give them to the client to hand-carry.
That is one way to solve your problem. If you are in an illegal state or the midwife is not interested in utilizing lab testing on a regular basis, then it might be the best one. The other solution is for her to get an account with a local lab. There are several national companies who have dealt with midwives -- Quest is one. Sometimes setting up an account with them is as simple as making a phone call, even in an illegal state. There are often local labs as well. I use a lab connected to a teaching hospital in my area and have been very happy. They provide me with free supplies for tests, like the swabs themselves, test tubes, needles, etc. I can also send women to blood drawing stations that are scattered throughout the area if that seems more convenient.
Hope this answers your question.