Check your local laws first. I know in NYC you are limtied to what you can purchase at an authorized dealer (think FFL.)
Its not spray works best, its what you can do to bring the spray into action and follow up with a plan. Also expect to get some in your face, so tolerance to it will help a lot. Attacks hyped up on drugs, alcohol, or other sort of mind altering substance can be less prone to OC.
I carry Fox Labs 5.3 million SHU 2 ounce Stream type spray. I am on my 5th can. I replace it every year or every use. I only had to use it once against attackers, the other times were used to spray myself or to get replaced. I haven't been carrying it lately because I usually keep it readily available in my front coat pocket, so it is summer right now and it will look weird and feel hot if I wear a coat.
My back-up to OC spray during the cooler times of the year is a folding knife. No point in fumbling around with a purse when every second counts. I can have my OC out dispensed, and my knife out and ready in 9 seconds (5 seconds is actually spent spraying the OC.) Practice makes perfect. Don't expect to just buy a can and expect to use it if and when the time comes. Buy multiple cans and practice.
I only keep cone in my house because of fairly close quarters (Tigerlight), but for carry I go stream because it can sometimes be windy and less of it blows back than with cone and I distance myself from suspicious looking people. A good offense is a good defense.
I don't know who has good prices because I buy my stuff along with a few LEO's.
