How can I get user details from a website?

I want to file a libel lawsuit. I have been notified from the website that the person writing the review request to remain anonymous. What legal document will I need to get that person's identity?

I own a small business. I recently read a review of my business on a website I advertise on. The reviewer stated that I committed fraud by "scamming" a large and very well known local non-profit organization out of hundreds of dollars, and then proceeded to harass the organization when they tried to collect on the debt.

I think I know who wrote this review, however, the website that this review was left on says they cannot give me the identity of the reviewer for privacy reasons. Since the statements made in this review are not only false, but has caused irrevocable damage to my business and income, I have chosen to file a libel lawsuit. I cannot file this without the person's identity, correct? What legal document do I need to legally retain this reviewer's personal identity?

Alasdair Taylor's answer to How can I get user details from a website? (1262070001)

In the English courts, you could could consider seeking an order for pre-action disclosure, or issuing a claim against the website operator and adding the user as an additional defendant further down the line.

If you are in the USA (as your reference to dollars and the US English spellings suggest), there may be equivalent procedures.

In any case, if this is worth pursuing it is worth instructing a lawyer and getting specific advice.