There is a stickied thread in the Diaper Talk subforum that has links to some free sample packs, but those may not be the type you're looking for. Some sites have sample packs of 2 diapers (like the bambino and ABU sites, or some on Amazon, etc) but you generally have to pay around 10$ for each sample pack of two. If you're looking for a lot of different ones to try out, you could try searching on eBay. I've seen a few entries there that have multiple different types of nappies all in one place. If you're looking for a good starting place and want to try a lot of different brands, that may be the way to go.
Wearingclouds.com allows you to buy individual diapers of a variety of different manufacturers and style (Tykables, ABU, DC Amor, Cuddlz, Bambinos, MyDiaper, Rearz), just be aware diapers are frightfully expensive when buying them in such small quantities; but the rational it may be cheaper to buy a sample and find it doesn't work for you for $4, then be stuck with a whole case.