I was given a REI tent last year (half dome), which appeared not to be waterproof anymore at all (probably to old). Since I've really liked the tent in it's construction and details, I would like to ask if it is possible to replace just the shell. I live in Switzerland, which mean that I would be interested in an european dealer of yours. Does it exist ?
Unfortunately, we do not have the rainfly available for purchase separate from the tent. We do sell products that can be used to waterproof the rain fly that you have though! Here is a link to an Expert Advice article with all the information on How to Waterproof a Tent. There is even a video that walks you through the process. Hopefully this helps!
Bummer that the shell spray didn't work! Given that we unfortunately do not sell the Half Dome rainfly separately, you can try using a tarp or waterproof cover. While a tarp or cover could serve as a fix and not a perfect solution, it's still an option if you're only interested in a rainfly.
While we understand that it's less that ideal for you to start with a new tent entirely, Half Dome tents come with a rainfly that aren't sold separately. If you're set on that Half Dome tent, the next option is to purchase a new one by selecting the specific tent here. Since we do not have any European dealers, you can have the tent shipped to Switzerland if you purchase it through REI! Instructions on how to purchase a new tent and have it shipped to Switzerland are here.
If you decide to go with a new tent, take a look at our tent care basics article on Expert Advice. This article has a lot of great information with tips that can help with preventing the breakdown of the tent's waterproofness.
We are happy to know that you really like the Half Dome tent, @Romain, and hope our suggestions are helpful to you!