You can order these from sites like Chocoate Trading Company. A mix gives you a bitter/sweet final chocolate and including dark will make the job easier. You can use any proportions, but 50/50 works well.
Suppose you want to coat some truffles in chocolate...
You need a smooth kitchen surface that's not warm. Granite and marble tops work well. Plus something very flat like a fish slice that can be used to scrape and spread liquid chocolate on the surface.
You can do the following process over and over again - it won't hurt the chocolate, as long as you don't get anything else into it.
(The theory bit is after this section)
Mix the solid chocolates and melt them thoroughly. Best way is a metal bowl over a pan of water.
Dry off bottom of bowl (don't get water in chocolate - its fatal).
Tip 2/3 of it (roughly) onto your surface.
Spread it out thinly, wait a couple of seconds and then immediately scrape it all back together in a pile and mix it a bit quickly.
Then spread it out again and repeat process.
After say 3 goes, you get a bit of the choc from the pile on you finger and put it on your top lip. If it feels cool, its ready.
Stick the chocolate back into the original bowl that still has melted chocolate in it.
Mix well and then as quickly as possible start tipping it over your truffles.
THE ORIGINAL MELTED CHOCOLATE SHOULDN'T BE ANY WARMER THAN AROUND 35C, or all the crystals will melt and you'll have to start again. If its just warm when you put a finger in it, that's not far out.
NOTE: Its best to practice everything up until the truffle coating to see what happens and get a feel for the process. When the chocolate is cool in the top lip test, you can just leave it on the surface to see if it sets well. Or put it in the bowl, mix it up a bit and then put a few blobs on something to see how well it sets.
Until you actually put it on the truffles, the whole process is repeatable and reversible.
The truffles are best placed on some sort of metal grid (eg. cake cooling), in order to pour the chocolate over them.
Then just leave to set for 5 minutes. When they look like they are set, put them in the fridge for 10 minutes or more. When you take them out, the chocolate should be pretty snappy, even when it comes back to room temperature.
At room temperature it should not be tacky to touch and should only melt in the mouth, not the hand. If possible bring them out of the fridge into a dry atmosphere, to prevent condensation on the surface ie. not a steamy kitchen.
If you just let melted chocolate cool, it sets with a mix of crystal forms and will be quite soft, because a good proportion of the crystal forms melt at a low temperature.
However, there are some crystals which melt at mouth temperature (higher). These are the ones you want to give chocolate a 'snap' at room temperature.
You get them by cooling the chocolate low enough to make them start forming, but not so low that the lower melting crystals form. In the absense of thermometers, reheaters and other equipment, the lip test helps because when the chocolate starts to feel cool, there will be the right crystals present.
However, there will also be some of the lower melting crystals present. You get rid of them by mixing the cooled chocolate back into some of the melted chocolate. This raises the temperature with the idea of melting out all the 'wrong' crystals and just leaving the 'right' ones.
At that point, if the chocolate is cooled, the remaining crystals 'seed' the chocolate, so that hopefully, most of the crystals that form after that are all the right ones. ie. they 'copy' the crystals that are already there.
Its like tempering steel, but more fun.
Which gives you the melt in the mouth feel !