Reddit mentions of NECA Simpsons 25th Anniversary Series 3 - Stephen King - 5" Figure

Here are some things I'd get a kick out of.

Overlook Hotel Ashtray (The Shining)
http://www.stephenkingcatalog.com/catalog/overlook-hotel-ashtray-p-1921.html

Pennywise Funko Pop Vinyl Figure (IT):
http://www.amazon.com/Funko-POP-Movies-Pennywise-Figure/dp/B00C0EB9EU

Stephen King Simpson's figure:
http://www.amazon.com/gp/aw/d/B00KOKJBBA/ref=mp_s_a_1_1?qid=1443945663&sr=8-1&pi=SY200_QL40&keywords=stephen+king+simpsons&dpPl=1&dpID=51tTwDNgZ1L&ref=plSrch

"Stephen King Rules" t-shirt (Monster Squad not a Stephen King book or movie)
http://www.amazon.com/gp/aw/d/B00R5655NY/ref=mp_s_a_1_2?qid=1443945723&sr=1-2&pi=SX200_QL40&keywords=stephen+king&dpPl=1&dpID=414WLNiUhHL&ref=plSrch

Ka Tet necklace (The Dark Tower):
http://www.amazon.com/gp/aw/d/B002M26Q2A/ref=mp_s_a_1_16?qid=1443945800&sr=1-16&pi=SY200_QL40&keywords=stephen+king&dpPl=1&dpID=41QQVUv%2BDYL&ref=plSrch

Christine miniature (Christine)
http://www.amazon.com/gp/aw/d/B00JQLQOK4/ref=mp_s_a_1_1?qid=1443945998&sr=1-1&pi=SY200_QL40&keywords=stephen+king&dpPl=1&dpID=51y1QyEJZmL&ref=plSrch

Other things to consider might be posters for movie adaptations, or art pieces, stickers etc based on his favorite book. Redbubble would be a good place to start, or etsy.

As far as special editions are concerned, check out Cemetery Dance.
http://www.cemeterydance.com/Merchant2/merchant.mvc?Session_ID=f20753bc2c4a56c13cb19199690278c5&Screen=CTGY&Category_Code=kingbooks&Store_Code=CDP

I'd also recommend combing eBay for older Cemetery Dance books.

Hope this helps!