On my raspberry pi running Raspbian strech 2018-03-13
I find:
rpi3 ~$ apt list xdotool
Listing... Done
xdotool/stable 1:3.20160805.1-3 armhf
rpi3 ~$
So you should also find it. Be sure yo have the repository in sources.list
and update/full-upgrade it:
rpi3 ~$ cat /etc/apt/sources.list
deb http://mirrordirector.raspbian.org/raspbian/ stretch main contrib non-free rpi
# Uncomment line below then 'apt-get update' to enable 'apt-get source'
#deb-src http://archive.raspbian.org/raspbian/ stretch main contrib non-free rpi
rpi3 ~$ sudo apt update
rpi3 ~$ sudo apt full-upgrade
rpi3 ~$ sudo apt install xdotool
apt-get update
recently, @Sam? If not, try running that and check again. As Fabian says, the package definitely exists, so it must be an issue on your end. Have you been able to install other packages normally?