I haven't bought a cage like that in years, but Blackburn used to make some. If you need something ASAP you could always us a regular cage with slightly longer bolts and put a few washers behind it.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk