Relevant websites that attract the same target audiences as the advertiser but without competing with it are potential affiliate partners as well. Vendors or existing customers can also become recruits if doing so makes sense and does not violate any laws or regulations.
Almost any website could be recruited as an affiliate publisher, although high-traffic websites are more likely interested in (for their own sake) low-risk cost per mille or medium-risk cost per click deals rather than higher-risk cost per action or revenue share deals.