Where can I get some immunoglobulin a or interferon?

It's complicated. You can't purchase iga. Different types of interferon (ifn) are avaialble as medications to treat specific illnesses (alpha, beta..). Most of the type you don't want to "get some" IgA or ifn, you want to measure the levels of these things in your blood. This is a simple blood test that can be done at most labs.