This is the "purist" method. You can buy old Penthouse issues on eBay for $5–$20 each. Using a home scanner (or a scanning app on your phone like Adobe Scan), you can create your own for personal use. Under "Fair Use" doctrine, scanning a magazine you own for personal archival purposes is generally considered legal.
The Internet Archive hosts full-text versions of past issues that show standard formatting and content flow. penthouse letters pdf
By submitting, you typically grant the publisher permission to edit and reproduce the material. Note that most editions state they do not pay for letters. Submission Channels: This is the "purist" method