Who is funding IS?

This Newsweek article seems to cover it.

In brief:
They hold an oil rich territory roughly the size of Austria so I would imagine smuggling and selling the oil would be their main revenue stream.

Selling artifacts is another.

Taxation and extortion another.

Kidnapping for ransom.

Donations from sympathisers.

Fake Humanitarian Aidraudulent Charity donations.

Reportedly their revenue equates to ~$6 million a day.