Billie,
It's great to hear an update on your little Northwestern. Congrats on your success, and I hope things go well with the guppy
.
Mike,
You can just buy or catch a rainbow trout and freeze it for a few days (at least three) to remove most of the parasites, unless of course, you just want to feed live fish. After freezing, you can cut off chunks with a sturdy knife or tree pruning shears and feed them to the snake. Leave the bones within.
Rick