As recent as November 2017 it is possible to unlock additional outfits for some LESPs via a variety of methods. Conditions will vary depending on the outfit involved.
You can unlock a creature's outfit within it's status screen. Click it's name in camp and then click "Change Outfits" to access this menu. If you do not see the button, then there are none available for that creature's species.
From there, click an outfit and follow the basic instructions within the menu to unlock what you want.
If you really like a particular LESP, it is possible to commission a new outfit for it from Ignis.
The pricing will vary depending on current Ignis' workload and the complexity of the idea, but expect to spend at minimum 100 USD for nothing other than an outfit to be drawn and added to the game, with you listed as the source of the commission.
Also be prepared that Ignis may refuse an offer. Contact him via In Game Mail with your idea and he will get back to you when it is convenient for him.