Hi,
A little red or orange filter on the RF window is either an OKARO for the IIIc to IIIg (and I guess the II's) and for the earlier II to IIIb's an ORAKO. (I just hope I got that the right way round.)
As for getting them and other bits, invest in the little Hove Foto book about accessories for the Leicas and rummage around flea markets, posh camera shops and ebay etc. They (the little book and accessories) turn up all over the place, spotting them and then deciding if the price is right is half the fun...
Regards, David
PS I've often wondered why our friends in China who turn out those nice lens hood don't do the OKARO/ORAKO's as well...