I'm jot even sure how to answer this one. If you are over 18 then the drug store or adult "playtime" stores, if under 18, then I'm out of it XD Ask your school counselor, or talk to your parents because they need to tell you how to be safe. Have you seen Blue Waffles Disease yet? That'll get you to talk to your parents if so...Good luck!
First if you don't know you shouldn't be having sex. I do commend you for wanting to use protection. You can find condoms just about everywhere grocery stores/ target/ Walmart/ gas stations/ Walgreens and planned parent centers. Usually you can find them near the hygiene aisles ( some stores have them locked up, so you have to ask an employee for them).. Gas stations have them behind the counter.planned parent centers give them out for free.
Walgreens, CVS, Walmart, Target, Kmart, Grocery stores. Dont be afraid to ask an employee where in the store they are located. And if ur asking where they sell them, ur getting ready to be smart about sex. Good 4 U! I'd rather u ask, than to be too shy and naive, and think "everything will be ok"