There are several spark plugs that can be used with a Tecumseh engine, including Champion, NGK and Mega-fire. In order to find the right spark plug for your engine, you need the model number of the engine.
To ensure you purchase the correct replacement spark plug, it's helpful to have a conversion chart. There are many available online. Typically, these charts list the model number of the engine in one column and the replacement part options in another column, or multiple columns, by manufacturer.
Spark plugs are generally available at auto parts retailers and online. Be sure to check the return policy of each merchant before you buy because some parts are non-returnable.