There are pleanty of wifi phones that will work without an active sim. The sim card is just to make and receive calls, so if the phone needs one to just "turn on" you can get an inactive or unregistered sim, i have literally hundreds of these, you can get them from your local phone shop.
There are some other phones however such as the nokia 6301, which the Wi-Fi portion is STRICTLY for a service called FidoUNO, aka wifi calling, it will only turn on if you have a active sim, and have the fido uno option on your account, all calls go through the wifi system instead of the gsm network. So like I said before, there is also another phone cant remember who, but the browser is the only thing that will work on wifi, if you try running apps they will try to conenct through cellular data, so be warned
