A reputable radiator shop is your best option, the chemicals used to dissolve an old liner are nasty and it is a lot of work (shake, agitate, rinse, pick, repeat). If a radiator shop is not an option you can strip it yourself.
A kreem liner is cream coloured and is soluble in MEK (methyl ethyl ketone). A 10% MEK solution in Xylene will dissolve the liner.
A POR-15 liner is silvery grey and is soluble in the POR-15 product available on-line or at some Carquests (around here). Use some kind of media (chain, nuts, bolts, screws) when agitating the solution. Use "The Works" toiletbowl cleaner to remove residual rust if it exists.