Many of the bass brands now offer bass guitars for left handed people and the number grows each day. Often you might find that local stores tend not to stock many but can often order them in for you. Another option is to order your left handed bass guitar on the Internet, which is perhaps why you are here!
Playing a bass left handed is exactly the same as playing right handed other than the fact that you use opposite hands. A right handed player will typically pluck the strings with his right hand and fret a note with his left, for a left handed player it’s the other way around. You would pluck with your left hand a fret with your right.
Another option you have if you want a certain bass that is not available as a left handed version is to use a right handed bass but still hold it left handed. For this, you will need to have the strings on backward so that when you hold it the wrong way round you have the thickest string on the top. You will also probably need to pay a store to set it up for you as the bass will need to be set-up to be strung upside down.
Another problem you might find with holding a right handed bass backward is things like the pick guard or the controls will be on the top. I think you would be best getting a proper left handed bass guitar but just wanted to share with you how you would go about using a right handed one if you wanted to.