Highland Woodworking and Old Brown Glue both sell Milligan and Higgins 192 gm granulated HHG. It is generally accepted as the best all purpose strength for most woodworking. The higher strengths are used a lot by luthiers who need the strength and are usually working on relatively small glue ups.
The commercial pots like Hold Heet aren’t double boilers but are a bit expensive. It’s a good idea to start out with a kettle or crock pot and get used to the glue first. If you decide it is for you, then you can spend the money. The pots are certainly worth the money but only if you become a full time HHG user,
…….. And you will. :-)

The early bird gets the worm but its the second mouse that gets the cheese.