Their only options are Google, Bing, Yandex, Exalead, Gigablast, and Mojeek, and most probably in that order as the last two need to grow considerably first. Unless there's any others that have their own index?
They probably scrape Google. Willing to bet there is no deal. Google would never agree to such a thing. Even paying $XXXXX a month isn't worth it to them.
Yeah sorry, I was only counting engines suitable for DDG, ie. English language etc. There's also Qwant and although they are crawling their results still appear to be Bing.