Should I buy a new gas boiler.?

I have a service agreement with my gas company, which means that they inspect my boiler every year and do any necessary adjustments etc. But they have told me that, as the compahny who manufactured my boiler no longer makes the unit and any spare parts for it , I should buy a new one. For they say they would not be able to get any spares for it if it broke down. It's about 15-20 years old, but seems to be working fine, so I'm reluctant to replace it. I realise that a new boiler would be more efficient, cheaper to run etc., but uming I do not buy a new one and it breaks down, should a good gas installer/plumber be able to get hold of the necessary second-hand part(s) to get the thing up-and-running again?