Glocks have a HUGE after market industry in the USA. I'd say only 1911 type pistols as many parts & custom services.
Check out www.Glockworx.com www.glockstore.com www.Customizeyourglock.com
As many LE armorers & tactics instructors advise, Glocks do not need a lot of knick-knacks or gadgets to function. I would add a plug(with a Glock tool or lanyard) & 3 dot night sights(Trijicon HD).