Some places (web auction sites for example) might still have Windows XP for sale. After a quick look it seems Amazon has too. So you can very well find a copy to buy, if you indeed want XP (and why not).
In general, you don't "uninstall" an operating system, at least unless it's a virtual one (where would you uninstall it from?). So for any operating system, if you want to replace it with another, just install the other operating system over the old. In practise, either before or (if you can) during the setup program of the new operating system, reuse the partitions of the old system or if you can't (for example Windows and Linux require partition types of their own), remove the old partitions and create new for your new system. Usually there's a shortcut if you're not interested in the details, which simply uses the whole disk for the new operating system.
The only thing to worry about are backups, if you want to preserve some data.