I used to cut 100mg down and fool with the PG/VG ratio but have since been ordering from Wizard Labs.
They will supply 12,18,24,36 mg (or higher) premixed. Not only that, they will supply most PG/VG ratios also,e.g. the 80/20 that I like.
The prices are very good. I went throught the numbers, and fooling around with mixing 100 mg down yourself doesn't save much since the PG and VG are not free. Neither is my time to do this.
If course when you add flavoring the nicotine strength is decreased a little. But I generally add only a few percent flavoring so the nicotine strength is only reduced a few percent.
Here is a link :
http://wizardlabs.us/index.php?route=product/category&path=60
EDIT: NEVER MIND ! I just noticed that they are out of stock on many juices and they don't show anything but 100% VG or 100 PG now. I just ordered from them a few weeks ago and they had all the options described above. They recently moved so they may go back to normal soon. I'd check with them later.