We made our own. An article on how to do so was in an Aquarium World years ago and if anyone wants a copy I am happy to email it. Be aware, if you do not have basic electrical certification I suggest you get an electrician or someone who knows what they are doing. If you get it wrong and it overheats, shorts out or something and burns down your house, insurance will not pay out.
Our heat pad is not buried in the substrate, it sits under the tank itself. It is wire (like that used in electric blankets) coiled on Pinex. The wire just sits on the Pinex and was taped down with parcel tape. Because it is evenly spread it does not cause stress on the tank base.
Like Alan, we got Argus Heating to do the plugs and have them heat shrinked and sealed. Our heat pad has been running for over 10 years now. The only problem we had was when the thermostat died and we had to replace it. That is the hardest part now - finding a suitable thermostat. They only produce all in one heaterstats now.