Yes, you can purchase compost. Usually in bulk, but you will probably have trouble finding a nursery that sells it. Unfortunately, I don't think there is a great demand for it so it isn't that common. Most people who look for it are true gardeners who most likely will begin making their own soon after. Others who are not gardeners think compost is just garbage and stinks.
The composted cow manure in bags will work well, but as is the case with most bagged products, it's hard to know exactly what's in the bag. If you read the ingredient lists on the back of those bags you'll see that it says "not less than 51% composted cow manure". The other 49% is who knows. But it IS undoubtedly some form of organic matter and will help your soil for sure.
Best thing to do is learn about composting and start making your own. That way you KNOW what's in it.