Holt McDougal publishes solution manuals to many of their textbooks, including Physics, which can be purchased directly from their website. These can also be found at most book retailers, including Amazon.com, Barnes and Nobles and Scribd.
The most direct way to find a solutions manual for Holt McDougal Physics is to simply purchase it from the company that produces the corresponding textbook, which in this case is Holt McDougal. This also helps ensure to that the proper edition is being purchased and that the answers correspond to the questions. Solution manuals can usually be purchased online from more traditional retailers, but this adds to the risk of getting out-of-date or incomplete editions, especially if ordered in digital form. As of 2015, the Holt Physics Solutions Manual for grades 9 through 12 is priced around $50 on their website.
The course instructor for a given Physics class can often provide the solutions manual. Sometimes the instructor includes copies of portions of the solutions manual, where appropriate, as part of an online resource. Inquiries can be made to the course instructor to see if such a resource is available, or you might attempt to find other courses utilizing the same text online.