I must be about the same size as you, I swim in most XLs and standard larges aren't long enough.

As Newracer said, "tall" or "long" is the key.

I really like the Gap classic polos in LT. It can sometimes be hard to find the color you want in stock, but they are worth looking for. I buy them online.

Old Navy has tall sizes too, but their fabrics are generally really thin which is bad for concealment.

I'm still looking for tall Tshirts that are thick enough.

Let me know if you find any other good brands of polos or Tshirts.