I went through this recently and found a number of threads on EGullet. Some people suggest that Horlick's is better than Carnation, so I tried it and still didn't get much flavor. Still others suggest using CTL which you can find here but before I ordered that I read about barley malt syrup and since they sell it at Whole Foods, I went out and bought a jar. Very much improved and probably about the best you can do. I don't recall the brand and I'm not where I can check the jar now, but it's along with the honey, agave, and other sweeteners.
I still have yet to get the malted powder to make these truffles. I did call my local brewery who also referred me to merchants who sell products to make your own beer. I am going to look online for the malted powder because I do not see myself traveling an hour to the next nearest big town in my area. I have been in no big hurry anyway because it has been raining and spring snowing here all month...but these truffles are definatley on my to do list.