Yes, another diffuser would work, as long as it isn't one that heat them up.
I would not use any other brand of essential oils for this kind of use than the oils form Young Living. You are looking for oils that for sure are 100% therapeutic grade.
It's the only way you can use them on your skin and in your body knowing they are not harmful.
Young Living for example only use plants grown on their own plots, that have been free from all kinds of toxins for at least 50 years.
Here's a good link if you want to read some more.
http://neatoilessentials.com/essenti...ssential-oils/
Also, yes, you can blend your own thieves oil, but you're not guaranteed the same results as of the "official" one. It all depends on what oils you're using.