If you're a size M, you're in luck on Amazon: https://www.amazon.com/Arsenal-Crest-Polo-Shirt-Puma/dp/B01HU169HY/ref=sr_1_2?ie=UTF8&qid=1470159949&sr=8-2&keywords=arsenal+fc+polo
Otherwise, eBay? Or World Soccer Shop has these ones on sale (I have one, and it's quite comfortable): http://www.worldsoccershop.com/78706.html
I'm gonna add a shameless plug for smile.amazon.com here. Adding the smile in front while shopping on amazon turns part of Amazon's profits into a donation to charity. It doesn't cost any extra, and you can pick your cause.