As far as inexpensive options, there are a number of choices depending on how inexpensive you need them to be.
The least expensive are probably the Xiaomi brand, which can cost as little as $10 each. They are zigbee, but not actually zigbee certified, so they can be a little eccentric. But there are a number of community members using them successfully.
You can ask more questions in the following thread (this is a clickable link), those are the people who are using the Xiaomi devices.