You can get them direct from Gerbing in Kent by mail order.
They're very helpful when you call.
DO check sizes and tell them your exact chest size, 'cos the jacket liners seem to come up large. You'll also be best to acquire a Heat Controller thermostat thingy.
Using Gerbing's recommendation of adding +4 to your chest size and +2 to your arm length to arrive at their sizing gave me a jacket liner that was as tight as a straight jacket.
I should have taken a 48 chest x 36 sleeve according to Gerbing's and ended up with a 52 tall x 38.