Got most of the parts off ebay - mostly shipped out of China and takes 2-3weeks to arrive but everything has arrived without an issue. Stuff is cheap enough I ended getting 2 of everthing in case I screwed something up building it. Pictures below are for reference when looking for the parts
Valve - must be "fail closed" type that opens when energized. Search ebay for "12v solonoid valve" and you should find some.
Flow Switch - switch should "close" on flow but all that I saw were this kind . Search ebay for "water flow switch"
Temperature Switch - This needs to be a switch that is "normally closed" and "opens" when the temperature gets hot in 35-40C range. I used ones marked TLRS-9700 with a 40C rating. Search ebay for "bimetal temperature switch"
Switch - this is a SPDT: ON-OFF-MOM type switch which was hard to find at a decent price but DigiKey Electronics - Electronic Components Distributor
has them. part is 401-1363-ND
I am currently adding a timing relay to cut-off the circuit if the valve is open more than 20s (as the recirculation does not stop if I forgot to turn on the hot water heater - not that that ever happens). It is optional and adds a bit of electronics to an otherwise simple circuit. But here is what I have on order - again go to ebay and search for "DC 12V delay relay delay Turn on / Turn off switch module with timer" I will use the delay-off functions. They have lots of other cool relays that will work as well. I can't vouch for this bit as I haven't got mine yet.
Other plumbing bits and switch plates you can source from local HW and RV store. The temperature switch I embedded (glued) though a fitting - but probably better to just have a short piece of copper pipe and strap it to the side of the pipe and insulate over it.