The feeler guage from Sears that I use is probably twenty years old, but it has inches and mm marked on every blade. The blades come in thicknesses of 0.001 increments, so you don't get nice even mm values on every one. I would expect they are still available that way.
I use the closest one to the mm value and err when adjusting valves to the large side (I will use a blade that is a half thousanth or more large rather than one that is a half thousandth or more small). Works fine. Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)