Here's how I was able to rent long-term, easily, without any garantía (in Santa Fe province):
NOTE: Before you begin this process, make sure the owners/rental agency are willing to accept Locativa as a guarantor (sometimes referred to as seguro de caución).
1. Go to Locativa (designed for anyone who doesn't have access to a garantía):
www.locativa.com.ar
2. Fill out the info on the site and wait for them to contact you (in my case, it took 48-72 hours)
3. Pay at the office (you might be able to do this remotely, but I just walked in and paid with my credit card from my US bank in a single payment)
4. Locativa will have you sign a contract where you confer them the legal responsibility for the garantía, and hold a USD deposit from you for doing so, which will be returned to you at the end of the rental contract in physical USD.
5. You then take the contract from Locativa to your rental agency and have them create the rental contract where Locativa appears as the guarantor, which you then sign to get the keys.
I had a great experience with them -- professional, quick service, and helpful employees. Without Locativa, it would have been practically impossible for us to rent down here with any option outside of Airbnb, which would have easily cost 4-5x what we now pay.