Condoms keep sperm from coming into direct contact with the vagina. They also help prevent the spread of STIs. However, they must be used correctly every time you have sex.
The best thing about most types of condoms is that they help protect you against sexually transmitted infections (STIs), including HIV. Lambskin condoms, however, are the one type you should not rely on for STI protection—they are able to block sperm, but not infections.
You have to make sure to use condoms correctly, every time, no matter what, in order for them to be effective.
You can find them just about everywhere, from truck stops to supermarkets, and even online.
If you're allergic to latex, you'll need to use a non-latex condom, or try another method.