Any fabric shop that is worth your business should have a shirting section. You can just walk in, ask where you can find the shirting, and pick out something you like.
When shopping online, it can be harder to get a good idea of what you're looking at. In the absence of touch, it's good to know a thing or two about the different weaves of fabrics. The weave is how the fabric was woven, and this determines not only how the fabric looks, but also how it will feel and drape.
Pinpoint is the work horse of shirt fabrics. Typically they produce a heavier and thicker . and thus more durable . fabric. and thus more durable . fabric.
Pinpoint is a less formal look than broadcloth or twill, but more formal than oxford. This, combined with its durability, make it a great choice for your everyday work shirt.
Chambray is simple weave, similar to broadcloth. It is different because it uses a much heavier yarn, and typically different colours for the weft and width.
Technically a twill fabric (look for the signature diagonal texture) denim is produced in much lighter weights than the ones your jeans are made from. Those are the ones you can use for a shirt.
There's a catch though. Numbers above 100s are typically made by twisting two yarns together. Like a 160s thread count means that two 80s yarns were twisted together for the thread.