Somewhere such as banggood - https://m.banggood.com/Aqara-Zigbee-1_2-Version-Window-Door-Sensor-Smart-Home-Kit-Remote-Alarm-Eco-System-p-1149705.html

Usually grab them on sale. Also a few others from the Aqara family such as climate (humidity and temperature) that you can get cheap. Have a motion sensory from them too that works ok but i don’t currently have in use.

I combine these with a Conbee II and in home assistant I use ZHA (over deConz, which is an option too) to manage connectivity to the sensors. I don’t use the Aqara hub any more as I’d rather run things locally via home assistant than using a third party hub which removes any potential concern around privacy. I’m honestly not sure if these sensors are upgradable or not but they work reasonably enough. Maybe once every 6 months I need to spend 2 minutes reconnecting one but it’s not too common. It helps to have some ZigBee smart power plugs scattered throughout the house, even if you aren’t automating power to things, as they form a mesh network which can make battery powered sensors more reliable.

I picked ZHA over deConz largely in the basis it’s development was linked to home assistant so I figured over time it’d see more development from the home assistant devs.

I aim to use ZigBee where I can over WiFi or Bluetooth devices. Lower power and more responsive in my experience. Also frees up the wifi traffic and the more ZigBee things you add the more reliable the mesh network gets.