Are they safe if you use a condom?
Nothing is 100% safe, but they are certainly safer with a condom. It also depends who provides the condom... if their cock is presented sheathed - how many before you have sucked it? IME most GH users don't use condoms.
The only place to find them in adult stores?
No. They can be found in clubs and venues, as well as in public places
If you were going to suck your first cock would you find one for it?
No. I'd go to a club - get the feel for the scene more. A one-on-one with someone you've talked to and got to know will provide a much better and safer experience. Mutual respect and trust is important.
Please, ignore all the "fantasy stories" you read about GH - 99% are just that, fantasies.