I have a clip on mic for use with a chromatic tuner. I imagine it would work with an autoharp. I bought it in the Early Music shop and he demonstrated it by clipping it to the frame of a Zither, so I imagine in would work for an autoharp. Search for "clip on microphone" in an on line shop.

Correction, you may need to search for "clip on pickup" so as to avoid wading through dozens of tie clip mics.

Looks like this