Proof of independent self-employment ? (Istanbul)

Hi there,
I work as freelance translator and just found the work of my dream - international company with head office in Amsterdam looks for freelance translator of Russian. All my experience and bla bla bla are OK and pretty suitable for them. The only thing is they ask for -
Proof of independent self-employment and/or all required tax documentation -precedence will be given to candidates who are available immediately and able to provide proof of self-employment and/or tax documentation within 48 hours;

Does any one have any idea what kind of documents I can(?) get in Turkey for this?
I have a long term stay permission and local tax number.