Ammonia and water are fully miscible. They dissolve into each other easily in all proportions, so you don't really need any special carrier molecules. Higher temperatures will drive ammonia out of solution into the gas phase, but at 60F the equilibrium solution is still over 35% ammonia by mass, so you really don't need to worry about water not being able to hold enough ammonia on its own.
What you may need is some method of buffering pH, as small quantities of ammonia dissolved in water-based blood will make it strongly basic. The fundamental cellular chemistry of your world may well be adapted to more alkaline conditions than we are, but they still probably won't appreciate the swings in pH produced by using up ammonia. Chloric acid or carbonic acid could by used as buffering agents, leading to ammonia being transported partially in the form of ammonium chlorate or ammonium carbamate, both of which are also highly soluble in water. Carbonate, sourced from carbonic acid, is a buffer in terrestrial organisms, and some molluscs neutralize ammonia as ammonium chloride (not chlorate; the chloride in this case is a neutralizing agent for ammonia excretion, not a temporary buffer agent for ammonia transport).