In one sense, getting the River Song look is harder than with other characters, simply because she rarely wears the same thing twice. However, the good news is that almost everything she does wear, she seems to make her own, and I’m not just talking about the astronaut suit.
So with River, it’s a case of doing an image search under her name, and finding the outfit you can most easily get the bits for. Then all you need to do is get the hair right (otherwise you’re on a hiding to nothing) and commence strutting. The strut is a good 40% of who River is. She’s never knowingly outfoxed, rarely flustered, and blessed with catlike charm. And she’s a little older than the Doctor’s usual companions, and far more assured.
The easiest River outfit is probably the all-denim affair from The Impossible Astronaut. None of these clothes should be hard to come by, and again, once you’ve got the hair and the poise, you’re done. Then there’s the army camouflage gear from Flesh and Stone, that’s not going to be too hard to track down.
So, here’s a slightly fiddlier costume to aim for, and one which is as close to the definitive River Song costume as it’s possible to get….