Filters for Agfa Optima 1a?

Hi all - The other day I was really tickled to find an Agfa Optima 1a at the local vintage/antiques mall. Twelve bucks, with the leather case and teeny box (attached to the strap) holding the flash. I'd been thinking about adding a simple 35mm to the stable, so it seemed like fate. The meter works, the shutter seems snappy, etc. And did I mention it was only twelve bucks?! (Yes, but it bears repeating, IMHO).

Now I'd like to get at least a yellow filter for it - the manual says it takes S 35.5 filters, but I'm having no luck finding these. EBay, Amazon, B&H, KEH, etc., all turn up bupkis. Well, except for some lens covers in that size.

Any recommendations for a source? These cameras were so plentiful that it seems odd that filters would be scarce.

Thanks in advance!

If you want a new one, still listed at
-) B+W
-) Heliopan

I would look at rummage boxes.

35.5mm yellow filters turn up on eBay now and then, but not often. Alternative solution is to get a step-up adapter ring for the next larger size of yellow filter you do find - for example, a 37mm or 40mm yellow filter. That's what I use on my Agfa Optima.