After we got our tandem, we also started looking for matching jerseys. We ran into 3 problems: fit, cost and design/pattern.
We found one answer at the local sporting goods store. They sell workout/exercise clothing made of wicking materials. You can even get them at Wal Mart. This gave us matching T-shirts which we wear around the neighborhood, on training rides and to casual events at a reasonable cost.
We also looked into making our own and developed a pattern that had the seams where we needed them to make fitting easy.
An easy way to improve the fit of even T-shirts is to turn them inside out and put them on. Then pin the shirt where needed to make it fit the way you want. Take it off and stitch it. My wife also put short zippers in the front of the T-shirts which is a nice touch and helps. It gives an opening for air.
Good luck!