not sure if this helps but..
here is a flexible one for $6.95 which has a male one end female the other. flexibles are available in many lengths and arent terrible priced compared with others i've seen.
http://www.sweetwater.com/store/detail/Gooseneck13/
look at page 2 of this pdf. it looks to have some sort of extension rod which is somewhat like what you wanted. cant vouch for a price though.
http://www.allprosound.com/catalog/pdf/specpdf/Atlas/clamps.pdf
why is 5/8 27tpi hard to find? because it is non standard and is only used in that industry because manufacturers profit from it.
if you can find a 5/8 27tpi tap you would be better off buying 5/8 pipe and tapping it yourself. this is what you would call the cheap option if a bit labor intensive. i know you would rather buy pieces pre-made however that is going to cost you much more in the long run. you could even buy two taps and make your own adapters. it might actually even be cheaper to have a machine shop use a lathe and make you some adapters and thread the pipe for you then to buy it online from recording industry manufacturers.
they do make 5/8 27tpi to standard adatpers.
here is a cheap option i found
5/8 27tpi to 3/8 16tpi adapter for only $2.99
http://audio-video-supply.markertek.com/broadcasting/Thread%20Adapter
you can buy 3/8 16tpi threaded rod at lowes! $5.60 for a 72" piece
http://www.lowes.com/pd_44603-37672-11019.0_0__?productId=3128769&Ntt=threaded+rod&pl=1¤tURL=%3FNtt%3Dthreaded%2Brod&facetInfo=
ugly perhaps.... but cheap!