The prison commissary sells envelopes, pens, paper and stamps. They are reasonably priced. If the inmate is indigent, there are provisions made to gift these items to them.
https://www.inmateaid.com/ask-the-inmate/how-much-does-a-envelope-cost-and-how-much-does-a-stamp-cost-and-do-you-have-to-buy-paper-an-a-pen-why-cant-you-send-them-an-envelope-and-stamps-do-they-mark-up-supplies-that-much-where-they-have-to-sell-envelopes-an-stamps#answer
Accepted Answer Date Created: April 26,2017