OP didn't return, but I remembered to ask today, and here's the answer for anyone else who might be interested...
The TriPark Pass is produced and managed locally, and is not available on the national website. You can purchase the pass at the front gate of any of the three national parks covered by the pass... Hawaiʻi Volcanoes, and Puʻuhonua o Hōnaunau on the Big Island, and Haleakalā on Maui. For a gift, just inform the ranger and don't sign the pass. The person you gift it to will sign it, and show ID to use it.
To purchase the pass remotely, call the parks fee office at 808 985-6151 with a credit card and the address you want it mailed to, and mention it is for a gift and they will take your order.