Not sure where you are located, but around here it is easily found in garden stores. The stand-alone garden stores have more options than big-box stores do. I would check the brand websites for mercury information before buying, though. Otherwise, you can buy online from gardening and hydroponic shops.
Fish is a very good source of protein; high quality as it is meat based. I would be very careful about the source--feed grade vs fertilizer grade, that sort of thing. Google feed recipes, Late one night I tripped across some mixes in England where they still feed fishmeal. It gave very specific information, like making cookies! If you can't find a specific recipe,remember, a little goes a long way because it is a high quality protein.
Fertrell is well known for their feed ingredients. Here's a link to what they offer. You can try their "Locate a dealer" page on their web site. Usually you can find their products at a feed store or feed mill. Feed mills mix up feed recipes for people out of the ingredients they sell.