Ecigsmokestore has a great variety and in all the popular strengths. Dawn (the owner) is a great lady and I have yet to see better customer service.
There is a loose rule of thumb on strengths in a PPD (pack per day) method:
1 PPD lights or less - 11mg
juice or lower
1 PPD regulars - 18 - 24mg juice
1 1/2 PPD regulars or more - 24mg juice or higher
There will be a testing period in which you become comfortable with the product and find the right dosage for your cravings. From personal experience, I was a 1 1/2 to 2 PPD smoker full flavor and I started off with 36mg juice. I've stuck with the 36mg and haven't had (or wanted) an analog since vape # 1.