Dear Diego,
Amazon, AbeBooks, and Preserving Christian Publications all are a wealth of used Catholic books - you have to be patient and find them though. I am talking about used old Catholic books - not re-printings like the ones done by TAN (which are a great blessing as well).
I bought a book of Massillon's sermons and selected works in French published in the late 19th century for only a little bit over $20 on Amazon. You never know what you will find.