This is a somewhat belated reply, but in case you (or anyone else) are still interested in this question, there are a number of options that I know of ...

- Ren fairs: (I mean, people go there dressed as D&D creatures and call it Renaissance, so Narnia would probably land well within the acceptable range of costumes, even if your costume is... a dryad). 

- LARP events : Live-action roleplaying events are another great place, especially the groups that are aiming for a fantasy theme. Different larps have different norms, but you might be able to get away with, say, a King Peter armor set. (They typically require weapons to be within an approved spectrum of safety, so you might want to find/craft a padded/foam version of Rhindon. Or get a greatsword and go as a Centaur!)

- Around the house : Preferably if you live alone... or have a very accepting family who don't mind armored knights or centaurs thumping around in the kitchen.

- Making fan films: Whatever costumes you have or can make, in whatever environment you choose! (And feel free to share it online!) 

- SCA : The Society for Creative Anachronism is a massive medieval/Renaissance group that does what could be considered 'soft-core' reenacting. People can really dress as almost anything so long as it's vaguely medieval, although you'll find that many members are aiming for a much more historical look and feel. It's just not subject to quite the same level of scrutiny or focus on being accurate to a specific historical period as with a reenacting group. A suit of Narnian armor would probably slip in undetected, especially if you emulate Peter's armor from the movie, which is really a mix-and-match of historical armor pieces from the 14th and 15th century. (But like LARP, swords and other weapons are subject to safety restrictions if you plan to fight with them). 

- There are some churches and schools doing Narnia plays, so if you really wanted to you might be able to volunteer with them. Community playhouses might also be an option. 

- NARNIA-THEMED PARTIES ARE DEFINITELY AN OPTION AND MUST BE EXPLORED AT ONCE! (I also think it would be awesome if a Narnia tabletop RPG was developed... a game party centered on that would likely also be a good place for your outfit). 

I know the pain of being an adult with too many costumes and too few costume opportunities, lol.  But I guess cosplay photos are always a possibility...

I also find I can reuse costumes. For instance, my hobbit trousers are also my leprechaun ones, the stripy leprechaun socks are used in a pirate costume; the long waistcoat I adapted from a blouse for VDT Lucy became Bilbo Baggins' one with a different shirt.