I'm planning on cosplaying River Song at Eleventh Hour next year, and really need help with the wig.

Ref picture: http://rohan-prakash.com/wp-content/uploads/2011/09/River-Song.jpeg

I've been looking around for ages trying to find one that works, and I've found this one:

http://www.wigstoreuk.co.uk/desiree-blonde-curly-p-671.html?osCsid=9c04f2732adf95572e9ae538e2c5b0f9

Only problem being? It's far too light, and the site doesn't offer any other shades, so if I was to get it, I'd need to know how to darken it. Sorry if this has been asked before, but I really don't know where to start!

There are tutorials on dyeing wigs. They more or less say the same thing, this being one example: http://firelilycosplay.deviantart.com/art/Sharpie-Wig-Dyeing-Tutorial-156134465

But really, it's probably more trouble than it's worth. I know you've looked for a long time already but I'd keep trying if I were you, since River Song's colour isn't something too unusual, there's bound to be the right wig out there.

Thank you for the link! I may have a go at it, though most likely as suggested, I might just keep looking - I don't trust myself to not mess up a wig, and I don't have £30 to throw in the bin, lol

I second Pez, but if you do go down the dying route I recommend acrylic inks over Sharpie pens. There's a greater colour spectrum and there's no messy breaking open of pens.