I've heard nothing but bad reports about the FF8 gunblade. There have been two or three versions offered at this point, and all of them suck.
The buster sword has appeared in at least a dozen versions, and while one or two of them were close to being accurate they are indeed supremely heavy. The one pictured above is...ridiculously inaccurate. It fits nothing from the game, or the movie Advent Children.
The only Final Fantasy related sword that's available online through, say, Amazon for example that I can vouch for is the Sephiroth Masamune replica. It's under 100 bucks, at 68" it's ridiculously long (though not as long as the game version, since that would be somewhere around 7-10 feet) and it's overall a nice piece. This is the particular one I bought,
http://www.amazon.com/Fantasy-Children-Sep...1300577-4260067
And since Amazon shrinks the images so horribly, here's a pic of mine that I posted on Flickr.
Visit the photo's page for larger sizes.
Sephiroth replica masamune sword, Advent Children | Flickr - Photo Sharing!
A friend of mine found a fairly good Buster sword replica for sale at a renfaire. Another Flickr photo,
http://www.flickr.com/photos/animakitty/299743262/
I'm not 100% sure, but it seems to be this model,
http://www.amazon.com/Final-Fantasy-Buster...1300577-4260067 and it isn't bad, accuracy-wise. Right shape, somewhere near the right size, but from what my friend told me it *is* extremely heavy.