Sure you can buy something from the store ASAP
can be found at Walmart near the bandages. Usually the lowest shelf...YSMV
Since we know you have 5ml of 12mg/ml, or 60mg of nicotine, and you want 8mg/ml we can use simple ratio math of 60mg / 8mg/ml to show you need 2.5ml of that USP VG you just bought from Walmart to make 7.5ml bottle of 8mg/ml diluted juice.
I'm not going to say it will be good or exactly what you wanted, but since it is quick and from a local store, plus gets you to 8mg/ml, it meets all the requirements of your post.