they are standard tactile switches. they are about 6mmx6mm base with 4 legs - they are in pairs so make sure you solder them the right way. Easy to check with a multimeter.
The only thing you have to watch is the button height. they come in various heights and if you are not sure you can simply trim the long ones if you are carful but i am sure someone with a xenarc can measure them....or i will tomorrow if i get time to take one apart.
Every electronics store sells tactile switches. You are jsut gonna have to be careful desoldering the old legs - get yourself a solder sucker.