TN Visa Under My Own Company

I've recently been asked by a past client to do a bunch of work for them again. I'm in Canada they are in US. Hopefully this will be understandable makes it harder when I remove company names.

I recently started my own company it is Incorporated in Alberta. I work for my company and get paid a salary. The client has asked me to work through one of their existing contractors as it is apparently painful for them to start a new contract directly with me. I have seen the amendment to the contractors existing contract with the client that now includes my company as being approved to work through them.

I of course will need to get a visa, I am a Technologist and need to work under an Engineer. Should I be getting a letter from the Client or the Contractor. There is an Engineer at both the companies that I would be working under, Maybe I should just get both?

In the past I have also had a letter from the local company/management that states such thing as my salary. Do I personally do this letter and sign it?, I read something about not being able to work for yourself I don't know if it applies in this instance or not. Another wrinkle is my company name contains my name.

Any answers would be appreciated.

Thanks

Let's remove the term "contractor" for the US agency you will be working for, taht will simplfy thins.
So there is a client, that is hiring you, thru an agency.

The letter must come from someone other than yourself or your company, of course.
The TN is being sponsorrd by the either the client or the agency, your (or their) choice; it doesn't really matter. If the agency sponsors you, it gives you the added freedom of being bale to go to other clients thru them without needing new TN (as long as the terms and job duties are the same).

They will sponsor YOU, not your firm, regardless of how you are paid.