I've recently posted a wanted thread on retrobike for the same thing which came up with nothing. After much research I ended up buying one of these: http://www.sjscycles.co.uk/ird-defiant-mkiii-7-speed-freewheel-shimano-compatible-prod22876/
They're not cheap, and you want to avoid the Mk1 & Mk2 versions because they had shed loads of problems, but this one seems to be well sorted. It appears a really well made bit of kit and is (allegedly) a copy of the old shimano freewheel you're looking for. It indexes well enough with my DA 7402 8 speed levers.
I've recently posted a wanted thread on retrobike for the same thing which came up with nothing. After much research I ended up buying one of these:
http://www.sjscycles.co.uk/ird-defiant-mkiii-7-speed-freewheel-shimano-compatible-prod22876/
They're not cheap, and you want to avoid the Mk1 & Mk2 versions because they had shed loads of problems, but this one seems to be well sorted. It appears a really well made bit of kit and is (allegedly) a copy of the old shimano freewheel you're looking for. It indexes well enough with my DA 7402 8 speed levers.
Hope this helps