There's no need to purchase expensive extension cables. I've been using 10 gauge outdoor landscaping wire from Home Depot for years. It's two conductor similar to a large gauge lamp cord, is UV/weather resistant, etc. Will Prowse gives it a thumbs up as well.
You can connect to your panels via MC4 connectors or you can use marine grade weather proof butt connectors with UV resistant shrink tubing. For extra protection, I covered mine with corrugated wire protective tubing. From a brochure I found online, it appears that Thermo King/Thermolite uses a form of "residential solar connector". There's no magic in using these but you can probably find them for sale online once you're sure of the type. Personally, I'd cut them off and go the MC4 route.