There are very few after market spoilers available for the 5th gen legacy. You can always go with the "M3" style mini-lips or the EZ-lip flexible style ones, but for a true after market spoiler, theres pretty much only two options for us that aren't like $800 and have to be imported from Japan.
I've actually posted about both of them in the Exterior mods thread, gimme a sec to go an grab the quotes.
Edit: Here we go.
The nicer, pricier, more subtle option:
And the more affordable, more noticable option. Keep in mind, we haven't found any known 5th gens with this one yet, as it's a fairly new option, so buy at own risk:
Originally Posted by Humble Rumble
A new raised trunk spoiler option has been found by forum user testa422 (he doesn't come on here much so I figured I'd post it). It's a similar design to the one in the previous post, but is only one piece (no bottom piece) and a bit of a taller design. Looks to be about halfway in between the above spoiler and your typical STI-style spoiler in height. Also has a 4th brake light as well.
This is currently the only picture on the listing. I have contacted the seller seeing if they have pictures from other angles.
You can either get it painted or unpainted, and both options are very reasonably priced, quite a bit cheaper than the spoiler in the previous post. Also stated to come with mounting hardware, template locator strips, gaskets, and instructions.
Unpainted ($84.99): http://www.ebay.com/itm/142204520504
Painted ($136.99): http://www.ebay.com/itm/142204511196
There's also a few 5th gens that have made various STI spoilers work, the look is down to preference. Here's one with the spoiler from a '15 STI:
And here's my own vehicle with the spoiler from an '04 STI set on top of the trunk to see what it looks like (my friend got an extra spoiler by mistake):