Mackay did list some hoses for older Citroens but they have mostly been deleted from the catalogues. eBay might turn up something NOS, but the usual European vendors are probably your best bet of finding the correct hose now. cx-basis would give you the part number and a price benchmark to begin with if you don't want to buy from them.
The hoses on a CX are much simpler than the fitted, multi-branch 'octopus' type hoses you find on most later models, so you have a good chance of finding a longer hose of the same diameter you can cut down to fit.
Try Ian Steele at Just Cits near Maleny. Just google and you will find some club references with phone number including from the Citroen Club in Queensland.