I checked Ryde website, https://www.ryde.nl , a search for CSS only found some references to history.

I would be surprised if they are made again.  I suspect that SJS found some new old stock.

There was a lot going on during the change from Rigida to Ryde, not just the name, but new owners, new manufacturing location, acquired another brand, rationalised products to avoid duplication... I read that the CSS machinery never made it to the new factory, and was sold for an entirely different purpose. I don't think CSS rims were a huge commercial success, I think touring cyclists are just as likely to need a replacement for other reasons than wearing one out. Then of course, along came disc brakes...   

It works, but it isn't undefeatable.  I used it when I first got my Raven, the entire purpose of that bike was to be a super reliable commuter, for a job where being late caused others considerable inconvenience.  No punctures for 8 months, 11,000 km, then a shard of glass stuck in the tyre didn't give the tube chance to heal.  By the time I'd come to a stop, there was green slime everywhere, the messiest puncture I've ever had. Examining the tube later, it had sealed 4 or 5 previous punctures. I swapped out the tyres for Marathon Plus next time they needed replacing. 
One of the issues with self sealing tubes is you're unaware of it happening, so whatever caused the damage might still be in the tyre and repeat it.  There may be better sealants available now, I think Thorn recommend the Schwalbe one.  On my utility E-bike, the only repair kit I carry is a seal and inflate aerosol (Vittoria Pit Stop, though there are others) I've never had cause to test it, though I bought it after being impressed by someone else using it. It sounds a risk relying on it, but I'm never that far from home on this bike.
For leisure riding, I'd rather choose the best tyres for riding and accept the punctures, though a couple of years ago I had a spate of them that tested my resolve.

Thanks John, appreciated.
I was greeted with looks of surprise and ‘it’s a no brainer’ when I was talking with fellow cyclists re my recent puncture-fest ; made me think I’d been missing out on something pretty obvious.
That said the other cyclists were all MTB riders and not tourers like me; different rims, tyres and surfaces.
As I’m not that fond of rear tube punctures when I’m loaded up and have to defenestrate the kitchen sink to fix, it I’ll experiment with injecting some slime into an existing (schwalbe) tube and see how it goes. If you hear cross-pond cursing you’ll know the result! 

I have no connection with this ebay seller, but you could get yourself  a UK manufactured Thorn Nomad?
Thorn Nomad | eBay UK https://share.google/dFiSinkrOe9OVBnQf

Wanted Thorn Shepa for a 5ft 10ins male
Living in Bournemouth and ready to travel to collect.
Message here or email
Robbie.Caudle@icloud.com

Cheers
Robert