Or to be completely offline, you can flip a coin 256 times to make the private key and have a calculator and pencil and paper to calculate the public key and address. I want to do it one of these days just to show that it's possible.
I was just about to reply with dice when I saw the table-top game dice.
Is there a better source of entropy available to the average person that could be used to mechanically generate a private key?
According to dice ware and other sites, a really good dice would be "casino" grade dice. Those are usually 6 sided dice.
You just have to use an unbiased method to roll the required number of bits needed. Like roll one dice, 1-3 = 0 and 4-6 = 1. But that's too much work and you'd roll 256 times. Better is to roll 2 or 3 or several at a time to get bigger values (not added together, but representing base 6?)
Personally, what I would do is just use those dice results as a seed for a cryptographically secure random number generator. You'd still need to roll the 6 sided dice about 100 times to equal 256 bits.