If you want to stick with stock coils then I'd highly recommend the Vaporesso iTank. Their GTI coils are the best I've used, specifically the 0.2ohm variety.
a little more info needed.
what smok mod and tank have you been using and what is your prefered wattage used. MTL or DTL
as its a smok im guessing clouds so as linked above the vaperesso 80w or 220w kit but if your mod you have now is suitable then just get the (imo) tank of the year so far the vaporesso i tank. buy in uk comes with 2ml poxy size but also buy 8ml bubble glass for under £20 https://www.ecigarettedirect.co.uk/clearomisers/e-cig-clearomizer-tanks/vaporesso-itank
bubble glass at bottom of post, make sure you pick 8ml