It depends who you are and what your Internet connection is. Usually you rent it from your ISP or hosting provider; prices vary but it should be less than $1/month/IP. Note that you don't own these addresses; you're just renting them from the ISP that owns them.

If you are an ISP (which I'm guessing you aren't) you can get them from ARIN/RIPE/APNIC/etc. or buy them which is a more complicated topic.