TCA Srl is a small Italian engineering firm which has been making glow-plugs and other model racing accessories since 2005. They have a comprehensive online catalogue of plugs in a variety of heat ranges to suit any application. A speciality is mega-durable plugs specifically designed for combat use. I have independent confirmation that the high-performance plugs are also used successfully in F2A speed models. The company also make high performance heads for modern engines such as the Fora 2.5 cc units.

If you're into maximizing the performance of your glow-plug engines regardless of age, a plug of the correct heat range is an essential component. Here's a sample price list (in euros) for the company's standard glow plugs (the kind that the "average" flier would use) as of early 2017:

The TCA online catalogue is well worth a browse. I know from experience that inquiries to the company are attended to very promptly. Payment is by PayPal and goods are dispatched very quickly following payment.