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=merino

Also peruse Wool-underwear.com at
http://www.wool-underwear.com/storepro_usdcart/agora.cgi

Sue