Merch is a tricky thing, in my eyes. I’m all for officially licensed merch and/or anything else authorized by the artist. Posthumous estates can be trickier.
That said, I have seen all sorts of Rowland items.
Here are some official ones:

The Reuss RSH-03 is replicating Rowland S. Howard’s basic pedal set-up, combining the sound and circuits of his two 1970s pedals into one compact unit. The pedal offers true vintage sound and response thanks to meticulous research and the use of real, vintage new old stock parts. The pedal is endorsed by the Rowland S. Howard estate who contributed to the development.

As longtime fans of all things Rowland S. Howard, HOLMES is pleased to offer up a run of custom straps based on the style and specs of the last strap Rowland used on his Fender Jaguar. The strap is a classic skinny with a sliding shoulder, buckle back adjustment, and features stamped lyrics and artwork sourced from Rowland’s notebooks of the 1980’s.

Corey Duffel’s new pro deck is a tribute board to honor the legendary musician, Rowland S. Howard. To celebrate this release, Foundation has a created a microsite featuring a video with Duffel, photos, and a Duffman Jams Rowland music playlist.
Luckily the Foundation microsite for this sold out skateboard was saved by ArchiveOrg on the web, and you can find it here.

An anthology celebrating the life of Rowland S. Howard.
The imported Six Strings That Drew Blood is a deluxe vinyl 4 LP set in a deluxe box, with each LP in its own sleeve, along with large format 16 page booklet. Mastered for vinyl, pressed on audiophile quality 180 gram virgin vinyl.
Compiled in conjunction with and with the full support of Genevieve McGuckin, Harry Howard and Mick Harvey among others.
Here’s some items that I found to be interesting.

Saint Rowland S. Howard, 8″ glass jar votive, Saint of the Gloriously Gloomy on Etsy.