Employment Lawyers Can Answer Your Employment Law Questions
The agency cannot force you to do so but that may mean you not being able to continue providing services to this particular client as the client has appointed this other agency as the sole contracting point.
You may ask the client to reconsider/waive this issue and see if they are able to contract directly with you.
May I help further?
I am afraid the agency cannot be forced to do that, unless the client agrees with the agency that they should do so, or unless the agency agrees to do so with you.
All the best
I am afraid I am not aware of any such websites. Perhaps you may search for the same on a search engine.