Joint signature for renting (Mexico City)

Hi! Im trying to rent an apartment but everywhere ive been asked for a "aval" (joint signature) with a property here in df. Im a foreigner and obviously i know no one who would be willing to lend me such thing. Has anybody been in the same situation? Ive hearf about some companies who "lend" their documents (afianzadoras) but im not sure if its reliable. Could anybody hel me?