I don't want to kill one (I'm even squeemish about stripping a dead one). Is there one laying around anywhere, not including the Markarth ones or the Whiterun one in the display case? I would especially like the gray Hjaalmarch one.

Thanks.

You may need to kill them.

Yep. Kill 'em or hope for a dragon attack or something similar that does it for you.

Some dragons actually have guard armor on their corpses... but as with most things in skyrim it's hit and miss.

I usually get a full set from the dragon attacking the whiterun watchtower. Otherwise you could look around in the guard houses of the towns/cities or if your pickpocket is 100% with perfect touch you can steal a set.

Do this before doing the civil war as MOST guards will be replaced with imperials or stormcloaks.

For Riften armor, I know for sure there are some dead gaurds beside one of the watchtowers on the road to the main gate.

They are already dead... a note explains the circumstances.
Maybe exchange for some gold (just throw a few coins into their inventory) if it makes you feel better, then go for a swim to "wash" the gear before equipping?

I'm not 100% sure but I have this vague memory that there may be a spare suit inside the tower, either on a table or in a chest, however that could be dead wrong.

Aside from that, you could try snooping around guard quarters in various citys, you might get lucky.

If your pickpocket skill is high enough, you could always do this:

http://www.youtube.com/watch?v=gjJ4moPxWGU

I'm thinking OP is a do gooder, or else he'd already have nicked the ones in the display cases.

By the nine, it's great being a criminal, you rarely have dilemmas such as these.