Any Auto Supply store, any good hardware store, but a feeler gauge for the valves may or may not work well for your plugs.
Depending on the plugs which may or may not have more than one "finger"... a flat gauge might not work well. Plugs typically nowadays use a wire-type gauge.... but, most modern plugs do not need setting.
Typical BMW plugs last 20,000 or more then replace.
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