Soymilk powder is becoming more common, and can even be found in some mainstream grocers, and at times in bulk. If you are having trouble locating it in stores, there are several brands available online, such as Better Thank Milk and Now Foods. Use equal parts soy milk powder in recipes calling for milk powder.
Ingredients, processes and products are subject to change by the manufacturer at any time. All products should be considered at risk for milk cross-contamination. Always contact the manufacturer prior to consumption.