Those gaiters are not available as spares and never were...
I've salvaged a few from otherwise irreparable rams in the past but they're rare...
I'd be looking for a generic steering rack gaiter that will fit... Measure what you need and look through catalogues to find a suitable replacement. I cannot offer any potential ones, it'll be a case of doing a bit of detective work... eBay listings sometimes give dimensions and I used this method to successfully source an inner CV boot for an Activa with an odd-sized driveshaft recently...
Welcome both to the forum and to Activa ownership

I've just ended some ten years of continuous Activa ownership and know them well. Rather too well I fear!!! There's not much I've not done on them
This page from my website shows how to repair the traditional leaks in Activa rams:
http://www.eastment.net/ramrep/index.htm