Does it really matter here? The address space is 2^160 (mapped from the 2^256 approx private key space). A little bit of bias in the RNG is not going to make very much difference (and a lot of bias would be pretty obvious ... two headed coin anyone?)

I got a few d20 somewhere (I'm of that generation...), but you could do it properly with d6. Just roll four times (ignoring values 5 and 6) and treat the results as two bits of the byte . Slightly more efficient than tossing a coin (not much), but you need to be pretty good at binary to hex conversion (though if you're doing the EC by hand too, that's the least of your problems...).