Anywhere. Any auto parts store. Just tell the knuckle-dragger behind the counter that you want the 180 that fits in place of the stock 195. You also need the rubber gasket that goes around the rim, and the fiber gasket.
After you get all three parts, drain a small amount of coolant out the drain in the bottom of the radiator. The face of the radiator towards the engine bay, there'* a little hole (get under the car). A gallon will do. If you choose not to, you'll have to replace some coolant later.
Pull the 2 10mm bolts on the Thermostat housing (the upper radiator hose goes to it), you may want to disconnect your MAF and TPS harnesses to keep them from getting in the way. Pop the housing/hose off. This is where the coolant leaks out all over if you didn't drain it first. Pull the old thermostat out (the car better be cool).
Now stuff a CLEAN rag in the hole in the lower intake where the thermostat goes. This will prevent debris from getting into your water passages while you clean the old gasket. Clean the old gasket material from the outlet and intake without scratching the aluminum. I also like to lightly sand both surfaces with 400 grit wet/dry. Clean it all up good, install the new rubber seal on the thermostat (taper down....like a cork), install the new fiber gasket (no rtv needed but you can if you want), and put the housing back on.
Very simple really. I suggest buying Chilton'* or Haynes' manuals first. They're cheap.