The main seller of merchandise for most of the Guide’s duration has been ZZ9 Plural Z Alpha which has lots of members-only merchandise available, including a nice towel with “DON’T PANIC” written on it.
Having a look on eBay will probably turn up some exclusive memorabilia, from towels to lifesize cardboard cutouts of Douglas’ mother drinking tea.
NECA are currently producing merchandise based on the Guide movie - action figures (from 3” Vogons to 10” Marvins) to plush toys to Marvin’s Heart of Gold gun. They also made some limited edition film prop replicas of Vogon stationery available - these may or may not still be available when you read this. Click here for the NECA HHGG page.