It’s never too late to improve your soil! Where to get your compost? I recommend your local nursery but you might be able to get it at a big box store. You’re looking for natural organic material and manure will work too. The ideal thing to do is to make your own with leaves, grass clippings and bits of plants you prune so you can control what comes onto your property, it’s cheaper and you don’t need to travel to get it but if that isn’t realistic buying by the bag of delivery by the yard if you