As a frequent purchaser of things from the Poulter line I can share this with you. His site over in Great Britian is www.ianpoulterdesign.com.
In there you'll see two sections; clearance and on sale. Both areas have products you can buy, with the best pricing coming from the clearance section, which are mostly last years styles. On the left side of the page there are also shirts available in the Tour section with his IJP logo.
I will tell you that right now his pricing is really good (make sure to click the USA flag for dollars), and you'll pay about $30 freight. That said, if you want to economize, get a couple shirts instead of 1.
The other place you can find his stuff is at www.zappos.com. Lots of new stuff arriving daily. Higher cost but no shipping. Both are good places to buy.
I will say the quality is similar to dress quality clothes (lined to the knee pants), great fabrics, stitching, etc.
I don't know who is Phil's tailor, and I have not seen any of his clothes available at Callaway or anyplace else. 100% custom.