I think the only place to buy the iPhone version is at the iTunes store. Unfortunately, each country has their own version of that. If Apple & SlingMedia aren't offering it on the Brazilian iTunes store, you're probably out of luck.
Kind of like watching Hulu from places they don't support, it won't work. Which completely bites, I might add.
This would be a really good time for the SlingMedia mods to chime in...
If your iTunes account is US-based, that should be no problem. Buy it, sync it, start slinging.
If not, there is a way but it's not so easy.
Be sure to check with Apple on this. My understanding of this could be out of date and invalid.
The iTunes store version you see is tied to your Apple account. Specifically, it's based on your billing address. For example, my Apple account is registered in the US. I can browse the UK iTunes store, but it won't let me purchase anything.
My family in England can buy stuff from the UK store, but then the purchased content is tied to their account. If I plug my iPod/iPhone device into their computer to sync the purchased content over, iTunes will warn me that my device doesn't belong to their computer. If I sync it anyway, it will erase my device and it will only be recognized on their computer. When I plug my device back into my computer, the same warning/erasure will happen again.
It's possible to copy the purchased content from one computer to another directly, then drag it into the iTunes library. This is allowed and legal, and gets around the erased device problem I mentioned above.
However, the copied content will not work until the password for the buyer's account is entered. You can do this because Apple allows up to five computers to share the same purchased content. Entering the buyer's Apple account password authorizes your computer to use their content, and counts as one of their maximum five authorized machines. Once authorized, your computer can use any of their purchased content.
In short, if you have a trusted family member with a US iTunes account, they can buy the app and authorize your computer as one of their five allowed machines. They would essentially be sharing their password with you. The app can then be copied to your computer and synced to your iPhone via iTunes in the normal way.
Hope this helps,