Can anyone tell me where I can find a replacement "bladder" or ink "holder" for a squeeze-fill English-made Osmiroid fountain or calligraphy pen that probably dates from the '70's?

Hello and welcome to FPN,

Any size 16 or 17 sac will fit most Osmiroid sac squeezers/lever fillers.

Sacs are available from many sources which is easiest and quickest depends largely on where you are, if you are in Europe or UK then Cathedral Pens, The Pen Museum are two in UK that are reliable and quick to respond.

If you are in US then there is an abundance of suppliers from any of the repairers who post here like Richard B, Giovanni at Tryphon to name just two. To fit the sac you will need some shellac which is a vailable from the same sources.

There are guidelines as to how to resac a pen in various threads in this forum, it is easy and straight forward on an Osmiroid.

Cheers, John