Depends on how you are going to power it. I got a 300mm×300mm 110v 600w with thermistor and adhesive. $21.00 usd plus shipping. daisyhuang@alirubber.com.cn is who I dealt with.
Ya around $20 for me also. What is your power supply you plan to use then go from there, remember AC powered is also a good way to go but requires a SSR.
mine cost 40 delivered via dhl
Ac powered is more efficient yes? Because no loss from ac/DC conversion? Maybe I’ll go that route… So if I ask for a “300 x 300mm 240v 600w silicone reaprap heat pad” will that be enough info for them? And should be no problem wiring the bed in parallel with the 12v power supply for the other electronics?
AC gives the advantage of not having to use a big power supply and keeping the heatbed current off of the controller board. A lot of the newer boards it doesn’t matter but for some it can be a bit much. 600W should heat up in 1 minute or less maybe. Then you will be waiting on the hotend to heat up instead of visa-versa. Determine the length of leads you will need then add another 50%. Cost is the same. Another option is weather you want it with adhesive already applied or not. If you dont really have another option of what to do to attach it i would get the adhesive already on it. There is a 3M paper covering it so it wont be sticking to everything until you peal it off . If you need holes in the heating pad make a detailed drawing and submit it before hand and get a conformation before the order. I had 3 holes put in my 1x1 pads for the bed leveling screws. You will also need a SSR. Get from Ebay or locally if you have such a store for that.
Cheers @Wayne_Friedt 
And the rest of you of course