All over Ebay. Here's a link to the OEM part. If you figure out just the right combo of search words for Ebay, (10mm chrome bolt cap) You can probably find a vender who sells bags of 5 or 10 at a time w/o the Honda number attached and they are much cheaper yet. I think I bought a few way back in 3 or 4 different sizes.
edit : Here's a quick Ebay find for 4ea at a time...cheaper. You can find extras of these for all the various sizes that Mother Honda put on our Runes, and even other sizes that didn't come covered, if you choose to add extra 'bling'. I found some matching little ones in a blue color with a Yamaha emblem on them that matched my 09 Vmax and it's color scheme perfectly too.
If you're talking about part item #15 (8mm)? I would buy OEM, not an aftermarket one. Many of the chrome caps Honda uses on the Rune are NOT off the shelf ones, and only Honda's OEM ones fit and finish as they should. They are of a bit different design. While many may fit a 8mm or 10mm bolt head, Honda's have a larger cap area or over all diameter to fit the space better or to fill the gap more completely and give a far better look.
If you ordered the OEM Honda ones (# 91456-MJ0-000) in post #2, while they are the wrong ones for the handlebar top bridge, they do fit other locations on the Rune. Link below and scroll threw related fitment and it shows where else those 10mm # 91456-MJ0-000 are used.