Some sources don't deal in "Tall" or "Extra-Tall", but in measurements. If you don't have someone to measure you, get something similar from your closet that fits, and measure that.
Try the Vermont Country Store at
http://www.vermontcountrystore.com/shopp...;keyword=merinoAlso peruse Wool-underwear.com at
http://www.wool-underwear.com/storepro_usdcart/agora.cgiSue