How do you find one? How much do they charge?
Some places have purchasing/procurement departments.
In my experience even if you have a purchasing/procurement department they generally do very little in terms of negotiating IT deals, they are just gate keepers because someone decided they should oversee all purchasing. They really only negotiate on products needed to make the product your company makes or resells.
Edit: I think I miss interpreted your question. I personally don’t take “contracts”. I’m just full-time at a company, I negotiated my salary myself when I was hired.
The closest I get to that is when I am pursuing an opportunity through a specific contracting agency that I've worked with for decades. In that case, I let them do the negotiating for me. That's a position of trust, though, that has built up over years and so is a bit unusual.