You can try mechanically "powdering" (grating or finding some other way to turn into powdered form) real chocolate (not compound chocolate) and mixing it in, if you really need it in powdered form.
You can also use compound chocolate if you want to use something more affordable, depending on whether or not compound chocolate tastes richer to you or not.
On using cocoa/cacao powder vs actual chocolate:
Real chocolate is made by combining roasted cacao/cocoa beans with other ingredients (normally sugar) and grinding them together to make a moldable paste (the real chocolate we know). If you mix 70 parts cacao based substances, plus 30 parts sugar (and other substances), then you get 70% chocolate. Or if you just grind 100% cacao, then you get 100% chocolate.
On the other hand cocoa powder is made by taking the cacao bean, and extracting its vegetable fat from it (called cocoa butter). The pulp left behind (normally in powdered form) is basically cocoa powder, which can be used for cooking and baking. Dutch processed cocoa powder on the other hand takes the pulp and processes it further (through the dutch process). Either way, cacao powder is basically cacao with its fat content removed.
On real chocolate vs compound chocolate:
While real chocolate is made of ground cacao beans, compound chocolate is made by combining cacao powder with some form of vegetable oil, palm oil, etc. Compound chocolate is what you normally see used by more affordable "chocolate" candies.
One of the main benefits of using compound chocolate however is that it doesn't melt as easily as real chocolate. Real chocolate can start getting real soft and melt on temperatures above 27 C. If you have powdered real chocolate in your mix, and it gets heated above that temperature, then you can just imagine the chocolate melting and clumping with the powder together.
On what kind of chocolate to use
If you're fine with the chocolate manufacturer's ingredients (which includes sugar and other chemicals) then you can look for n% chocolate bars (80%, 70% etc). If you just want 100% chocolate, then try to look for 100% chocolate bars, then just add some powdered sugar if desired (or other sweetener), to your mix.
As to chocolate taste, cacao beans from different geographical regions and different estates taste differently from each other (like wine). The way the cacao is fermented and roasted also affects the taste. In my case, the cacao I use from 2 different places produces 2 fairly distinct tasting chocolate bars.
In your case, I suggest looking for a manufacturer that produces the "right" chocolate taste for your purposes.