Extending Tourist Visa - Proof of onward Travel?

Hi,

I'm currently in Belo Horizonte and I'm on a Tourist Visa, 90 days. The opportunity has arisen for me to spend 2 months or so in Rio so I'd like to extend my visa.

The only problem I think I have is that I don't have a return flight. I arrived on a one way flight to Trinidad and Tobago and then travelled by land through Surinam, Guyana, Venezuela and then into Brazil.

I was trying to book a bus ticket across the border to Paraguay but the companies don't seem to offer this online.

Anyone got any ideas if I actually need the proof? or an idea of a very cheap 2km border crossing bus ticket?

I plan to continue around the rest of South America after my time in Rio.

Any help is greatly appreciated.
Thanks.