As the title suggests, where can I buy a flat top preban mid-legenth hand guard ,16" upper.

I like the look of this upper although I have not seen one in person.

It looks as if it has a slightly longer sight radius. True?

Seems RRA is the only one I can find on the net. I would like their although I hear about long delivery times and I don't want to wait 6 months.

Also, can someone suggest a sturdy flip up rear sight?

Thanks, I am a bit new to this.

Armalite and RRA both make mid-length.  Some of the kit makers like J&T or M&A might offer them too.  If you're looking at RRA, go through one of their dealers like Legal Transfers, Sable Co. or Adco instead of ordering directly from RRA.  They should have one in stock and ship right away.

The sight radius on the mid-length is longer than a standard carbine by about 2"

For a sturdy flip-up rear sight look at the ARMS#40 and the Knight(KAC)and GG&G sights.  The ARMS#40 is very popular here but all are quality sights.