Yes, you can buy a stainless steel oven/microwave combo without purchasing the rest of the kitchen appliances from the same brand. Kitchen appliance manufacturers typically offer individual appliances for sale, and you are not required to buy an entire set or other appliances from the same brand.
When shopping for an oven/microwave combo, you can choose a model that suits your needs and preferences, regardless of the brand. Stainless steel appliances are quite popular and available from various manufacturers, so you should have plenty of options to choose from.
Just ensure that the oven/microwave combo you choose fits your kitchen space and meets your requirements in terms of features, size, and functionality. It's always a good idea to read reviews, compare prices, and consider the warranty and customer service reputation when making your decision.