Rented Email Lists - vs Renting Newsletter space

or - as these are not exclusive options - the sensible thing would be to test all three:

1. send out your own emails to someone else's list (being careful A: to choose the right partner & the right list and B: to acknowledge the relationship strongly enough that recipients recognise it, and don't see your email as spam)
2. put ads (or - infinitely better - actual value-adding content) in partners' email addresses (yes, there are very many companies that do this & facilitate this)
3. as Jeff says, gather email addresses for yourself & target that list

closely track & measure all three. as with all of your campaigns, discontinue (or improve) the techniques that don't go toward achieving your goals & put your efforts into the ones that do.