Here in Malaysia, GTN can be sold without a prescription. But usually patients will get it free from the pharmacy if they are under follow-up in government hospitals. However, its a prescription medicine in UK if I'm not mistaken @mawit07. Not sure about practice in other countries though.
Actually your risk of getting heart attack is low if you do medical check-up every year and have a healthy lifestyle (diets, exercise and enough rest). However, be extra alert if you have a strong family history of heart attack even though you already living a healthy lifestyle.
If you are still concern about your risk of getting heart attack, probably its good to consult medical professionals. Cheers :)