hair thickeners

I like your name, I call one of my cats slinky kitty!

I think the only way any product can thicken your hair (meaning non hair dye products) is by putting a thin coat of something on each hair to make it feel thicker (usually silicones I think).  If you get any conditioner heavy in silicone that claims to add volume, it will likely do the same thing.  The problem with that is these silicone layers can build up, break off, and take your hair with it over time, and also, they can make hair appear limp or dull after they build up for awhile, so you may find that these products work great at first, then seem to make your hair go limp and dull.  That's easy to fix, you only need to use a clarifying shampoo once in awhile (I use one every 2 weeks) to remove the buildup so it doesn't get dull or chip and break off. 

So, my point is,  products with "cone" in the ends of some of its ingredients that claims to " add volume" or thicken hair are probably what you want to look for, and that clarifying is a good thing to add to your regimen if you're going to use these things.  Which products work best for you will likely depend on your own hair type (due to variations in the ingredients).  Sorry I can't give you a specific product.