You can make it yourself pretty easily. Many recipes and packaged dressings don't have much of the anchovy component, which is what you're probably wanting. The key is to use a recipe that has anchovies and an egg yolk. Otherwise its not going to taste like the real deal.
http://www.foodnetwork.com/foo...OOD_9936_27339,00.html
There's a pretty good one. It'll be good practice on making an emulsion too. Just remember to beat the egg yolks until they get slightly foamy, then stream in the oil VERY slowly. If it breaks, you might be able to add a little bit of warm water, beat it, and keep going. Just make sure you never stop whisking. I made some bearnaise last night and it didn't break on me (first try), so you can do it too.