Mark, I don't know how much or how easy, but you can get them from JMD.
If you send them the torch they can change the O rings and pressure test it as well if you like. True it is more expensive than doing it yourself, but I suppose it depends on how old the torch is and if you think it is worth getting serviced. (Servicing cost about £40)
Give them a call and talk with Peter (the torch man) and see if they can just sell you a bulb.
Underwater diving torch from JMD Technologies
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