Thin alu foil for ribbons ?

Try to get hold of some tinfoil wrapping paper that has wax paper bonded to one side. The alu foil is rather thin on this type of foil and the paper can be removed by heating with a hair dryer.

(Danish: Det er en form fo madpakke staniol med pap på den ene side - se efter det i supermarkeder som brugsen, jeg lavede et sæt bånd efter denne metod en gang i 90erne, det kan sikker stadig fås)
Wrigley chewing gum...

the foil wrapper on the flat "stick" gum is very thin. All that you need to do is to take a match to the paper and burn it off. You are left with perfectly clean annealed aluminum foil which is about 5 to 5.5 micron thick which is pretty good for a microphone or small tweeter diaphragm. Much thinner than about 3.5 micron and the foil is very difficult to work with.
Unwrap old paper in oil capacitor. Usually they use something around 5-6 micron (too thick for a microphone, but good for tweeters) and you can get long strips for cheap.

Note of warning--this stuff is supposedly harmful, so when you crack them open use lotsa ventilation, or do it outdoors.

Separate foil from the paper, lay it on clean piece of glass, clean both sides with cotton ball soaked in Isopropyle alcohol, and away ya go...

Best, M
Re: better option...

Much thinner than about 3.5 micron and the foil is very difficult to work with.

Indeed! I make living designing and re-ribboning/servicing ribbon microphones and deal with thicknesses of 0.6-1.5 micron. All I can say 3.5 micron is a piece of cake.

moray james said:
just in case PIO might just have PCB's I am not sure so donty quote me butto be on the safe side you can find/buy plastic film and foil caps to cut open and they are dry inside no oils. The larger the capacitance the thinner the foil will be as they are trying to pack the whole thing into as small a package as is usually possible.

All plastic film and foil caps I ever seen use sputtered layer of Alu.

With paper-in-oils the rule is the lower the voltage rating, the thinner the foil.