They exist.
I have seen such panels (5 1/4") with a couple of USB ports in them being sold in some local PC shops over here. They came with short usb cables meant to be plugged in on the mobo.
PS: Try eBay also if you can't find something like that.
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EDIT:
Since you also made your own audio splitter thingie, this might interest you to build your own front panel connections and combinations:
http://www.frontx.com/
They sell both pre-made panels, as (very interesting in your case) modular panels. eg: you can combine your usb ports and audio connections into one nice looking panel.
EDIT2: What menthix said about the cable lengths and grouping
So, make sure you check on that and then also check
http://www.usb.org/about/faq/ans5.