Like you, I recently purchased a CXS and C18. I really like them, and will probably buy the new impact when it comes out.
The first thing I looked into was the centrotec kits. There are none available in Australia, and the only option was Axminster in the UK to get the CE-SORT. I quickly found out they weren’t shipping Festool overseas anymore so I looked at Ebay and here in the classifieds section.
I was pretty active at looking for one and came across a couple of brand new ones on eBay US. The only issue was shipping and it made it not really cost effective for me. So eBay, gumtree and here would be a really good start.
Another option is to contact some Festool dealers. When I thought all hope was gone, by chance I stumbled across new old stock, a 2013 centrotec installers kit at one of the bigger dealers here in Australia. They found it when clearing some space in a store room. I ended up getting it for a heavily discounted price, and it certainly got me started with centrotec bits. It came with the older style centrotec screwdriver so I was very happy. Since then I added the countersink bits and I’m very pleased with it. I’ll slowly add as I go.
So if you’re keen, they are out there. Just got to be active in looking. I’m really hoping Festool bring out a installers kit with the new 18V impact, would be an awesome combination.
Good luck.
Andy