I see that Gourmet Vapors sells liquid nicotine (any other vendors?) and I want to raise the nicotine levels of my pre-existing 18 mg juices to 24 mg.
Any tips on how to do this are appreciated in advance
There are many ways - depending on the mg/ml of the other nic juice you want to add to it.
If you have some 36mg/ml you can add 5ml of 36mg/ml to 10ml 18 mg/ml and you will get 15ml of 24 mg/ml
If you have some 48mg/ml you can add 2.5ml of 48mg/ml to 10ml 18 mg/ml and you will get 12.5 ml of 24 mg/ml
If you have some 60mg/ml you can add 1.7ml of 60mg/ml to 10ml 18 mg/ml and you will get 11.7ml of 24.1 mg/ml
If you have some 100mg/ml you can add .8ml 100mg/ml to 10ml 18 mg/ml and you will get 10.8mg of 24.07 mg/ml
There are many vendors of unflavored nic - I have bought from Bluemist vapor and arctic vapor and liked what I got -Other have recommended xtremevaping and Box Elder Chemical Supply
PS - I did not do those calculations in my head I used section 4 under the vg calculator tab in Scubabatdan's e-juice calculator which you can download here:
https://www.filesanywhere.com/fs/v.aspx?v=89716a8d58646ea772a0