If you are trying to be a MWL purist, then the best thing I can think of is to take your coffee black. However, since that is not a helpful answer, let me point out that there are several milk-substitute alternatives to soy.
Now, none of these are great MWL, except for the rice milk I recommended yesterday, which is only 15% fat, and used as a condiment, is OK> And adding to coffee is a condiment. Besides rice, there are almond and hemp, for starters. You need to read labels, and be cautious about each of them, watching for added sweeteners and fat percentage. My husband, who doesn't follow this way of eating but has to live with me, likes hemp milk the best (and I said, "Of course! It's highest in fat!"). It has a very rich, creamy feel and a nice taste.
I also make a cashew milk for making DH waffles, sometimes. It is super-simple. 1/4 cup raw cashews to 1 cup ice-cold water, whiz in Vitamix or other blender until smooth. I keep cashews around in the fridge (so they don't go rancid--as an added bonus, I don't see them and eat them all) so I have quick access to a milk source for DH's meal needs.
Once again, you have to balance out how you feel about not being MWL on this point.
Also, if you haven't watched Jeff's lesson on label reading, have a look at this!
https://www.youtube.com/watch?v=gtxvIHRkoAg
