iD biscuits were very clever for their time, you could literally daisy-chain up to 30 devices on one cable and the panel would recognise all iD biscuits, or you could star them, or a mixture of both. They ceased to be made in 2004.
Yes you can get "modules" for some panels that will read and understand iD biscuits, just like UNG said, but what you can't get is new iD biscuits, so I would suggest you tell your customer the truth that if/when any of them fail you have to remove that device from the system, and it would be in the customers interest to re wire the system now while they still have a (old) working system, than for it to fail, and HAVE to re wire it. That way, you get to install a panel you are familiar with, and everyone will be happy.