If you just search eBay for "Toshiba Satellite P855-S5312 charger" you should easily find one that is suitable. Chargers are normally compatible with a range of laptops, sometimes a very wide range. The only things you need to be sure to check are:
- The plug that plugs the charger into the laptop is the right size. If a charger is advertised for your model it most certainly should, or you can send it back and complain.
- The voltage is correct, in your case I think 19V, but it should say on an identification label on the bottom of the laptop.
- The power in Watts (W) or the current in Amps (A), which again should be specified on the label on the bottom. (Watts = Volts x Amps so don't worry if only Watts or only Amps is specified.)
- The mains plug is suitable for your region.