Written determinations are not published. They are available on the IRS web site and in some research databases. Here are a links to a few access points.
A “letter ruling,” also known as a “private letter ruling,” is a written determination issued to a taxpayer by an associate chief counsel office in response to the taxpayer's written inquiry, filed before the filing of returns or reports that are required by the tax laws, about its status for tax purposes or the tax effects of its acts or transaction. Letter rulings are not published in the Internal Revenue Bulletin.
A technical advice memorandum, or TAM, is guidance furnished by the Office of Chief counsel upon request by an IRS director or an area director in response to technical or procedural questions that developed during a proceeding. The advice only applies to that specific case, but can still be useful for research. TAMs are generally made available after all identifiable information removed. However, they are not published in the Internal Revenue Bulletin.