There is a thread on page 3 of this section. I'm a numpty, so the link didn't save, but is says you can use a W033 housing from kilkenny castings. The thermostat is a TT1-160. There are a few brands, but common is Tridon. TT1 -180 is more likely to be in stock, but its a 180 F. opening instead of 160. Too hot!
Also I have had a new one not work, so watch for that! There are other brands on ebay, Gates, etc.
From memory the Flo head has a recess for the stat, so you will just need a standard thin gasket. 3 bearing heads have no recess, so a thick gasket is needed, with a hole larger than the stat diameter, otherwise the housing can be cracked by pulling it down onto the stat sholder.
Matt.