"rare" steering box ???

Are '70 Duster manual steering boxes really hard to come by ??

http://cgi.ebay.com/ebaymotors/ws/e...sspagename=STRK:MEWAX:IT&viewitem=#ht_500wt_0

I'm seriously thinking of converting to manual steering.. What's a good, used box worth these days? Does anyone make an after market one?

Thanks,

Kenny