AFAIK not available as spray, neither glyceryl trinitrate nor isosorbide.
Sublingual tablets are easily available OTC.
Dosage of the spray: 1-3 puffs - you should have discussed this with your doctor! If you need it regularly, switch to oral tablets.
It is pretty safe if you don't overdo it. Signs of overdosage: headache, dizzyness, rapid heartbeat, fainting.
Don't drive, it makes you light-headed.
Do not combine with viagra or cialis (and their generics), people die from doing this. 24 hours between nitro and these meds!
Altogether, you should discuss this med with your doctor, and you should discuss your BP with him, too. Because your BP is not safe.