I'm want to switch a AC heating element that draws 12A at 120V with a triac. How high of a safety factor do I need? Would a 20A triac work at 100% duty cycle? And sorry but I'm cheap and at mouser it looks like I'd pay close to $15 for one shipped... any ideas on what type of equipment might have a 20A triac in it that I could scavenge? 
Try one of the china suppliers such as eBay, they are only a few dollars and free shipping. Purchase several, you will not regret it. A 20 Amp will work great. When you drive it, use a solid signal, if you use an insufficant drive you may suffer from punch through and that will destroy your triac. 100% is no problem. Remember they turn off when the AC crosses 0. Timing is based on the zero cross. Use that point when turning on or off, if you do not you will get hash etc in radios etc. This is called EMI interference. When you start use something like 12 VAC, this will let you prove concept and be safe. When the voltage goes up be sure you have the proper knowledge and or help to do this, otherwise we may be sending flowers. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil
Are you trying to do phase control switching? If not I would use a 40 Amp SSR and low speed "duty cycle" switching.
https://www.mpja.com/40A-480VAC-Solid-State-Relay/productinfo/31933+RL/
https://www.mpja.com/Heatsink-for-Solid-State-Relay/productinfo/32052+HK/
A quick look on digikey, and I am seeing a ton of examples that are under $3/each, plus $5 for cheapest shipping option. That said, you're probably looking at $15 or so anyway from digikey, since buying just one of any part is a bad idea; always assume that you'll lose at least one, and burn out another one.
My attitude towards scavenging parts was transformed as I started making things in larger quantities than one-off's. It's not worth saving a couple of bucks on parts to use "whatever you happened to find" - better to use new parts that you can easily get more of (you'll get an email notice if anything you bought vaguely recently is getting discontinued so you have a chance to buy more) if you decide to reuse the design or something. My father still obsessively scraps boards for parts, and I used to as well... then I realized that except in highly unusual cases, the value of your time as a designer is worth more than you save on parts.