The majority of WW1 & WW2 European helmets used leather chinstraps, liners, adjustment cords, etc.
The sub-links from here will show the items that will fit German, Finnish, Swedish, Hungarian and Czech helmets; plus others too.
There are also a few non-leather items, such as US WW2 M1 Helmet parts for conversion to Airborne use,
such as the canvas/webbing A-frame. The matching leather chincups are also available to complete the conversions.
Details and photos to follow