If you search within ebay for - SCR motor speed control - you will get a lot of hits. Most of them are for 220V but a few are rated 110-220V. They are a 2000 watt unit with four terminals. Two are marked power in and two are power out. A search for - AC110-220V 2000W SCR Voltage Regulator Dimmer Motor Speed Temperature Controller - will find them.
The ones I use are rated as 220V 3800 watt and look like the ones in posts #3 & #21. They only have two terminals and the unit is simply inserted in the live wire. in series with the heater coils, but I don't know if they would work on 110V.