My boyfriend is 21 and wants to try viagra (sildenafil). Is there a certain brand? Or something safe we could purchase over-the-counter no prescription needed?

Not advised. Unless your boyfriend has erectile dysfunction (extremely rare for a man his age) he is not going to find a doctor to prescribe him viagra (sildenafil). Taking medications like this when you don't really need them can cause serious problems and even permanent damage.
No. There is no form of viagra (sildenafil) that is available without a prescription, nor should he take viagra (sildenafil) without discussing it with his physician. Every drug has side effects that can be dangerous in people with specific medical histories. Viagra (sildenafil) is not a simple drug to enhance normal sexual performance. It is medical therapy for erectile dysfunction. His doctor can determine if it is appropriate for him.