You can get spermicide form a pharmacy, it usually comes in different forms(gel, cream, foam, etc). It has an ingrediant that kill sperm called nonoxynol9. It can be used by its self but it is much more effevtive when used with another form of contraception such as BC, condoms, etc.
Heres a website with more info on how to use spermicide:
http://www.americanpregnancy.org/preventingpregnancy/spermicide.html
A LOT of people are sensitive/allergic to it, and it can be pretty irritating to your skin. That, in turn, can increase your chances of contracting HIV... so if you're at any risk for STDs it's most definitely not a good idea.
I have only used the foam, bc pills & patch makes me sick, rubbers hurt so much after childbirth so I used the foam spermicide I followed the directions exactly & it wked. As soon as I wanted to become pg again I stopped using it & the mo I stopped I got pg :) I never had any irritation although DH did a few times.
Always make sure it's in date:)