Where can I have lawyers bid on my assignment?

Lawyers don't bid against each other. Legal representation is not an auction. Lawyers charge by the hour or by a flat fee. Fees vary according to locale, and according to the experience of the lawyer. I believe you are being very simplistic--a cease and desist letter may be ignored or even lead to a countersuit. You bought these images. Did you copyright them? Did the people who sold them to you copyright them? Is someone due a license fee for YOUR use of the images? Have the models allowed others to use their images with or without the permission of those who sold them to you? Are the people who are using your images without your permission located in NYS or another state or out of the country? International litigation does not come cheap and your exclusivity may stop at the US border. You need to sit down with a lawyer and discuss all of these issues. My office gives a one hour free consultation. We quote fees during the sit down. You will find fees range from $200 to $500 an hour, and are less if a long term relationship is created.If you wish to learn more about our office, check out my avvo profile.

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LEGAL DISCLAIMER
Mr. Sarno is licensed to practice law in NJ and NY. His response here is not legal advice and does not create an attorney/ client relationship. The response is in the form of legal education and is intended to provide general information about the matter in question. Many times the questioner may leave out details which would make the reply unsuitable. Mr. Sarno strongly advises the questioner to confer with an attorney in their own state to acquire more information about this issue.

Mr Sarno is absolutely correct - what you think is "a relatively simple task" may not be as it is unlikely that a C&D will accomplish what you want with each of your competitors.

As you've been advised, legal work is never a "one size fits all" situation, as the facts of each case are different, so the rights and remedies are different as well, and very few assignments are as simple as a client may think they are. You need to be comfortable with whatever lawyer you hire and by the same token, your lawyer will want a fee agreement in place that specifies their scope of employment, so you should interview and meet with several to find a good fit and to reach a satisfactory written agreement .

Also, as has been noted, your assumption that a C&D will suffice is probably unrealistic, since sometimes infringers respond by saying they'll comply but don't, or sometimes say nothing at all and ignore the warnings.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.