Most “Laptop Chargers” will be of the power electronic type, with DC to DC conversation and PWM to achieve “12 V DC”. The better ones will have capacitors and inductors for smoothing, and be of the constant voltage type, which may not play well with sticking an potentiometer on the end.
Even if you go old school and buy an actual transformer, a bridge rectifier, you will still need “built in” electronics. Namely capacitors sized accordingly to smooth the full waveform into something that actually resembles DC.
If you’re interested in the challenge, then RS components and the like will sell you a transformer. Otherwise, just buy a cheap gauge master controller or the like.