There have been sites like that. MyBlogGuest, which was penalized by Google

There have been people here buying content for other sites. You could find the site you want to target, see if they allow guest posts, hire the writer, and then send it to the blog.
The market of guest posting and sponsored posts has been hit hard by Google. You have to get content like that posted on the downlow. This is only if Google ranking is important to you, which it is for most people. Too many sponsored posts linking back to your site is running on dangerous ground if you rely on search engines for your traffic. I've had customers who (I think) are posting my content to third-party sites.
Most customers are very specific on what they want, especially when they contact you privately. In other words, you wouldn't be able to put a link to a third-party site unless the customer specifies it. It would be awkward to approach a customer too about placing a link in their content. I think it would be better for you to find the target site and then have the writer write the content.
But, you could always contact Constant about it. Maybe they can work something out for you. I think they are the better contact than the writers.