Oveready takes surefire bodies, bores them and often does a cerakopte or custom HAIII on them. Very nice - about as nice a "standard" P60 host as you will find anywhere.
Elzetta - believe you will find these bored to 18mm on Oveready.
Ogaz / TNC products - WOW, custom made in USA (check the "surefire compatible" section) P60 hosts. Fantastic stuff.
The Solarforce lights generally need an outer spring on the P60 drop-in if it's a "surefire" P60 design. The Surefire design has more of an angle on the head - so a solarforce host is contacting at the top, less to no contact further down -so an outer spring is necessary. Consequence of this - if you are running a very hot drop-in the heat transfer to the host will be lousy. This is REALLY important in case you get a "Solarforce" designed drop-in - (pretty rare), I have one from Vinz (in Germany, not to be confused with Vinh in Seattle) - the sides of the drop-in are almost parallel - this means it will not fit in a surefire light - or one designed for surefire drop-ins. But in a Solarforce light it will contact & transfer heat well. Vinh has a video of wrapping drop-in in this copper film to aid in the heat transfer.
Believe there are alot of lower end hosts; trustfire, fandyfire, xxxfire, etc (all the name brands you do not particularly want to buy rechargeable li-ion batteries with their names on them).