There are several options which enable you to host BTCPay Server on your own hardware device. You can either build the device yourself or purchase it pre-assembled.
Once the hardware in hand, you can then select the service (some offer free versions) you'd like to use to download and manage your node:
Build or buy any one of the above nodes to begin using BTCPay Server on your self-hosted device.
BTCPay Server also provides instructions for setting up your self-hosted server from scratch, but this method is much more technically advanced. Find it here.