I just wrote emails to the local rescue groups and shelters and asked them if they needed volunteer photos. The first time I did this, I had no samples, but since then I send a link to some rescue portraits whenever I contact a new group.

You may find yourself overwhelmed with yeses though. I'm shooting for 13 or so different rescues now and combining that with my paid shoots keeps me pretty busy.

In this area, rescue groups are easier to work with than shelters. I assume that has a lot to do with regulatory red tape.