The part is BT1308W (and equivalent substitute: Z0103MN). You can buy these from digikey.com or mouser.com. You can also send a support ticket so that we can ship you a few spare parts. Do note that these are surface mount parts — unless if you have surface mount rework tools, it’s hard to take them off using a soldering iron. Again, if you send a support ticket, we can arrange to repair it at a reasonable cost.
There is a fuse on the 24VAC line to protect shorted circuit. I suspect that this damage might not be due to shorting but due to high transient voltages. OSPi 1.2 does not have built-in TVS diodes which are for protecting the triacs from high transient voltages.