Lotte,
You could start your investigation (and probably get enough material from them alone) from looking into just one company called EagleJet. The sole purpose of that company is to facilitate and act as a bridge between airlines looking for extra income or have some slack in their training department and newly qualified pilots willing to pay a commercial operation money to be line trained and increase their logbook hours on revenue generating flights.
However pilots are forced to pay in a number of methods from seemingly above board deals with airlines direct, however these pilots will often be charged well above industry standard for a Type Rating course. Indirect Pay to Flying.
The only airlines in Europe that don't follow these practices (at present and I hold my breath here and hope for the future) are the traditional flag/legacy carriers .... i.e. BA, Virgin Atlantic, Lufthansa, Alitalia, AIr France, etc.