What's the difference between pastels, oil pastels, and water soluble crayons and oil pastels? Pastels are chalky. Oil pastels are oily. Water soluble crayons are waxy. When using them, they feel a bit like a cross between oil pastels and regular old kids' crayons, only they're water soluble. Water soluble oil pastels are softer and creamier. You can blend water soluble oil pastels and wax crayons with a wet paint brush, wet fingertips (my favorite!), moistened Q-tips, moistened paper towels, baby wipes, etc.
If you are unfamiliar with oil pastels and don't know where to start, I suggest purchasing a mid-range brand. The super cheap ones are likely to be hard and difficult to manipulate. You can warm them up in your hand or under a light bulb, to make them more pliable, but I would recommend making the investment in a decent set to start with. The good news is that even the mid-priced ones are pretty good, so you don't have to spend an arm and a leg.
You can get oil pastels from any art supply shop and from the big boxcraft stores. You can also get them on-line from Dick Blick and Jerry's Artarama. I like Caran d'Ache Neopastels. They aren't too expensive, but they're still a bit pricey if you're just trying them on for size. Here are a couple of brands that are less expensive, but still of an acceptable quality.
Van Gogh Oil Pastel Sets (Dick Blick)
Sennelier Oil Pastel Sets (Dick Blick)
These are cheaper still. The colors are fairly intense and most of the crayons are soft. There are a few colors that are hard and waxy, but those can be warmed up before use to soften them.
Sakura Cray-Pas Expressionist Oil Pastel Sets (Dick Blick)
Faber-Castell Goldfaber Studio Oil Pastels (Dick Blick)
Water Soluble Crayons & Water Soluble Oil Pastels:
I'm a huge fan of Caran d'Ache Neocolors II. You should be able to find them at your local art supply store or you can get them online from Dick Blick or Jerry's Artarama. I’ve ordered from both and find that both have quick service. I think Dick Blick's website is easier to navigate and their prices are often better, though. Plus, I love their wish list options.
Caran d'Ache Neocolor II Artists' Crayons (Dick Blick)
Caran d'Ache Neocolor II Artists' Crayons (Jerry's Artarama)
Two other brands of water soluble crayons/pastels are Lyra and Portfolio. Lyras are about the same price as Neocolors II, but Portfolios (made by Crayola) are much cheaper. They are also softer and creamier in texture, so while they are good for thin, intense color washes, I find them unsatisfactory for thicker applications. If you're just wanting to give water soluble crayons/pastels a try and don't want to make an investment in something you may not like, I’d recommend picking up a set of Portfolios first.
Lyra Aqua-color Water Soluble Crayons (Artstuff.net)
Portfolio Water Soluble Oil Pastels (Dick Blick)
Other products that might be of interest:
Cretacolor Aquastiks (Jerry's Artarama)
Alphacolor Watercrayons (Dick Blick)