It's sodium metasilicate. Companies other than Red Devil make it, and it can have other names than TSP-90...
Look for phosphate-free degreasers... And then check the ingredients for sodium metasilicate. In California, I can get it in most small hardware stores as well as big box places like Home Depot & Lowes (near the paint supplies often, as it's used to clean walls prior to painting), Walmart, also online via Amazon, Ebay, etc.
Don't get hung up on the manufacturer or product name... As long as it's sodium metasilicate, you're good.
Edit to Add:
You can also just use regular TSP... Others have reported good luck with that, as well.