Alternative to NSA encryption algorithm I am looking for a strong alternative to elliptic curve cryptography. It should be something that could face quantum computing attacks, but nothing created by the NSA. 

I heard about isogeny key exchange, i.e. supersingular isogeny, is it really secure? Otherwise, what would be an alternative that fulfills the above?

In addition, what would be a post-quantum algorithm for file encryption? Perhaps snow 3g?
Supersingular isogenies are a rather recent attempt at post quantum security. You will have a hard time finding an efficient and secure implementation, and even if you write one yourself, the algorithms have not yet seen that much cryptanalysis. (Although that's a subjective judgement call.)

If post quantum security wasn't a concern, you could choose from any number of non-NSA elliptic curves. However, quantum computers, if practical, can break elliptic curves (using Shor's algorithm).

NTRU is, like the other answer notes, a more practical and established alternative for post quantum cryptography. It still isn't necessarily as efficient or well studied as elliptic curves, though. As a practical matter the main implementation is under GPLv2.

For key exchange there is also the "new hope" lattice algorithm which is being field-tested by Google as a possible successor of ECC in TLS. (Personally I find this to be one of the likelier algorithms to see use, but I would not use it – alone – for anything important yet.)

For file encryption, or anything that works with a secret key, you can use existing symmetric algorithms. You may want to choose a key size of at least 256-bits, though, to account for the quantum Grover's attack. SNOW 3G in particular isn't post quantum secure for that reason – it has a 128-bit key. Other standardized general purpose ciphers like AES-256 or something on the eSTREAM portfolio (Salsa20 at least has a 256-bit key) would work fine.