What do you mean by 'show cage'? One that offers no roll over protection but looks good? I can't imagine a roll cage that offers no protection would be in anyway safe as it will probably deform in a crash and squish your head.
There is a cage known as the Oulton Cage (as it's what the 197s they used to use as tuition cars at Oulton Park used to have) that come up for sale every so often. They go for about £100-£150.