I always get my shirts in the sales - anyone such as Debenhams, Next, Suits Co, John Lewis, etc. Got my latest from a small independent in Rugby - very good indeed.
Although I did have to buy one from Next in a panic - got to work to find that my shoes in my rucksack had rubbed against my shirt and put a dirty great mark just below shoulder height on the front. And I had an important meeting that day. So I had to buy an emergency shirt. Unfortunately it was an non-iron shirt and is a complete pain to get crease-free after washing.
However one thing is missing from life. If I want to give a diddly squat about the amount of pesticides and fertiliser used in cotton production, and the way that workers are exploited, and want a shirt in fair trade, organic cotton, I can only find one seller, who only does them in white. No hemp or bamboo mix available from anywhere.