Try to find a copy of Super Smash Bros 3DS. It'll update you to 8.1, which is supported by Gateway in the next firmware update (due in a couple of days).

 

For you to get the demo version of SSB you need to access eshop and for that the latest firmware is required. Your best bet would be to borrow a copy from someone close by.

 

You can also play it you know...

 

if i were you i would do the following:
just wait until gateway releases his update. If it supports 6.2 and you like the functions buy one
If supports a higher fw until 9.2 update your 3ds with a specific gamecard (too se which games have which fw see this: http://3ds.essh.co/)
If you dont want gateway with its fancy funtions and just play roms, update to 9.4 now and buy a sky3ds

 

Do you live somewhere near a gamestop? You can buy a used copy and return it.

 

just wait a couple more days to see if Gateway adds support for 5.x and 6.x. If you go back and reread their last post. they mention "adding support for lower versions every day". It's worth the wait and will save you $$

 

I don't know about your local library but all the ones by me carry 3DS games to rent/borrow. My library borrows them out completely free (unless you're late returning them haha). Could be an alternative if you have to update via gamecard