What natural emulsifier can I use for almond milk?
Your best bet for this is xanthan gum, which is an excellent stabiliser. Whilst the distinction between 'natural' and 'unnatural' is fraught with difficulties, insofar as xanthan gum is a product of microbial fermentation then it is no more 'unnatural' than alcohol or vinegar. 

Be careful not to use too much though (unless, of course, you  want your drink to have the texture of mucus), you really only want to use about 0.4% of the volume of the liquid you're trying to stabilise: so if you have 500ml of milk then you would use 2g of gum. Add it to your liquid and use a blender or an immersion stick to incorporate, a few pulses should do, any more than that and you'll just incorporate air and you could just end up with an annoyingly stable foam!