All airlines have individual interiors, I don't think there would be one airline out there that would have the same interior as another so the cost is small compared to buying a new aircraft in small batches, add in the fact that in most cases the companies already have the stock in the form of spares then overall its much cheaper. I would suggest to you that if you cant afford an interior they shouldn't be buying a 25 millions euro aircraft!!
Most of the airlines that have been buying the ex RYR are South American ones, several went to Varig etc. There would be a database on the net if you are that interested.