Online is best.
Under a cover, under the glove box. Plug in component, easy replacement.
The freezer plug in a 3.2 Isuzu Rodeo is a easy find. The freezer plug is located on the left side of the engine.
They are easy to find. Follow the spark plug wires down and pull of the spark plug boot. The plugs are right there.
remove the old spark plug and either take it into your local garage and the guy behind the desk will find you the replacement, or right down the type of spark plug (found on the side of the plug) and go to the NGK website and you will find the replacement on there, The above advice is if you have the previous spark plug, should you not, your best bet would be to go to the manufactures website for the engine and look up for engine failing that take the mower to your local garden equipment service engineer (yellow pages) and they will find you the correct spark plug.
NGK cross reference for this plug is CR8HSA
Crawl under any car and find the oil pan on the very bottom of the engine block. There you will find the drain plug. Usually near the rear of the pan. Very easy to spot as it is the only drain plug on the oil pan.
You could,
replace spark plug wires on a 99 sonoma
NGK spark plug part number BPR5ES
The replacement plug for Champion RJ2YLE would be B&S 799876. This spark plug should have a gap of .020. This type of plug should not be used though with Quantum series 625, 675 and 725 or any L-head engine.
Denso SK20R11 Iridium Long Life, OE replacement.
The company you bought the AC from should have a list of Dealers where you can get spark plug from. If not they should be able to tell you where to find one . If all else fails you can always search on line .