I agree that building your own list is the best way by offering good interesting content. I've been in the list management and broker business and even double opt-in names can trigger a spam complaint, something you do not want.

Also all legitimate lists are NEVER released to you. You pay per thousand emails deployed and the list owner sends out your creative. You receive the responses and those you own. I would be very concerned with a list that you can buy. Also most lists are for one time use unless you pay a higher price.

Also lists from good sources like responder files are expensive.

Take the longer, tried and true route and you will find that your customer/subscriber base will be much more responsive as long as you treat them like gold.