Fish Glue is a form of Hide Glue in that it is made out of the hide (or skin) of non-oily fish. Details of this glue is discussed in Hide Glue – Historical & Practical Applications. Fish glue has a couple of unique characteristics, one being it is more flexible and the other is it has an aggressive tack. In other words it is real sticky.
There may be more suppliers but one good one, the one I use is Lee Valley that stocks Fish Glue in sizes up to 34 fluid ounces (1 liter) and has a good shelf life of two years. Of course you can extend this by keeping the glue in a cool, dark place. The glue is a bit more runny that liquid hide glue like Franklin/Titebond Liquid Hide Glue (you have to scroll around on their site) or Patrick Edwards Old Brown Glue, which is an excellent liquid hide glue.
Many people don’t like liquid hide glue and prefer to use it hot. I like hot hide glue it works great in the summer time when the shop temperatures are high and the open time is extended. Liquid glues, have chemicals added to them to allow them to be liquid at room temperature. Liquid glues are very handy as there is no mixing and heating and can be used in a cooler shop with a good open time. The long open time is also another benefit of animal glues.
Because of the high tack of fish glue it can be used in conjunction with slower setting liquid or hot hide glue called glue stacking. This is where the high tack glue is used to secure an object in place while the slower setting glues take their grip. This is good for vertical applications where hide glue may not become tacky enough initially to hold a piece in place where gravity is a concern. But then gravity is always a concern.
First thing I did with the new bottle of glue was repair the sole on my reproduction Jefferson Booties. I have repaired another pair with liquid hide glue, glycerin and alum. The glycerin keeps the glue flexible and the alum makes it waterproof. I have also repaired shoes with just hide glue and glycerin and when it was dry I treated the exposed edge with alum and water. This in effect tans the glue making it waterproof. I tried this one sole with just fish glue, nothing added, as a test. If I have a problem I will add some glycerin for flexibility.
I have gone through three (17 ounce) bottles over the course of the last ten years and been very happy with the results. I use it for repairing antique furniture as well as new construction. I now have a big bottle (34 fl. oz.), which should do for a couple of years. I go through about a gallon of liquid hide glue and a couple of pounds of ground hide glue for the glue pot and hot applications, per year.
And of course this stuff is superior to modern white and yellow glues in every way.