As others have said, asking the restaurant is a good bet. I know where I live all the Chinese buffets have an option where you can get a take out buffet, they give you a Styrofoam container and you fill it up for $6 or so. I’m sure you could fill one with sweet and sour sauce if you wanted. The only problem is figuring out how long it will last. I have no idea about shelf life for warmed sauce. If the restaurant will sell you still cold or frozen sauce that will be better.
Look for Asian markets/Supermarkets in your area, perhaps with Google Maps. They might have what you are looking for.
What is the recipe for the hot yellow mustard they have at Chinese restaurants?
1/4 cup dry mustard powder (look in the spice section)
3 Tablespoons cold water
Mix, wait about 15 minutes for peak flavor. It will begin to lose flavor after that. Find your perfect timing to taste.
Yellow mustard is cut with things like flour to make it less spicy. Chinese restaurant mustard is just powdered mustard and water.
No vinegar?
I think we’re talking about the neon red stuff they pour over sweet and sour pork. But I’m not 100% on that.
Usually no.
I’m talking the neon red stuff we pour over sweet and sour chicken. 
Never say never, but I’ve never had it with vinegar. 
I just grabbed my French’s mustard, and the ingredients are vinegar, mustard seed, salt, tumeric, paprika, spice, natural flavors, and garlic powder. Mustard seed is still the second ingredient as one would expect of the primary dry ingredient in a liquid.
I think the vinegar is the primary taste that offsets the spiciness of the mustard seed. I’ve always thought of yellow mustard as primarily sour.
Chinese mustard is usually black or brown mustard seeds, and is just cool water & mustard and served right away, which is pretty much the recipe for maximum heat.
Most others have vinegar added, which mutes the heat, and also not immediately served, which also mutes the heat even more. Yellow mustard is made from yellow mustard seeds, which aren’t as pungent as brown ones to begin with.
I made South Carolina mustard barbecue sauce with a mustard that had brown husks in it. It tasted just like French’s yellow mustard.
Yes, vinegar is used in many prepared mustards, but she’s talking about Chinese mustard at the end, which is just mustard seed and water. The recipe for the classic Colman’s hot mustard on the side of the box is the same: Colman’s mustard powder and water. (The prepared Colman’s they sell in jars has a few other ingredients, and only a little bit of acid in the form of citric acid.)