As far as visual depictions go, drawings and paintings are our go-to. Though you likely can’t rely on the specific depictions of holsters as many had been painted much later than the period in question. But we can see the many varieties in which they were carried apart from military standard waist holsters.

Pistols were of course a secondary choice of cavalry and some other light infantry as rifles could become awkward to stow on a horse (and difficult to use at all for that matter). They were stored in holsters of easy reach on the side of the horse as shown in this 17th century English cavalry guide.

This was generally the norm until shorter carbine rifles became a popular choice much later, but by then the cavalry role started to change across many armies anyway.

Another example is indeed its naval use, with this gentleman tucking two pistols into his belt during the Battle of the Nile (took place 1798 - painted in the 19th century though apparently).

If you didn’t have a holster but wanted to carry a pistol, you were likely to shove them in anything that fit. Pockets in coats, bags, saddles, boots, and anywhere else where it would fit. I’ve read some references of “pistol pockets” before but I've found it incredibly hard to source due to the existence of the smaller “pocket pistols” type of pistol. I would imagine they were either a more civilian type of holster or a pocket in a coat designed to easily accommodate a pistol.

By far the most interesting thing are the flintlocks with built-in clips for universal carrying on belts or trousers. This would’ve eliminated the need for a traditional holster and could probably assist with concealability under a coat. Although I’m sure it wasn’t very safe.