I use the Antec 550W TruePower...costs about $100...but works great...
Yes you can buy an adapter, this is the first one I found on newegg (didn't look further):
In the review, one person said:
I purchased and Aspire X-Plorer (20 pin power supply) and an Asus A8E-V motherboard (24 pin power). When I try to play DVDs my system crashes. The Asus tech tracked the problem down to my power system, even though he wasn't aware I was using an adapter. Buy with caution! You might be better off in the long run purchasing a power supply with a 24 pin connector. I know I wish I had.
who knows if it's true or not...probably not...but...depending on what you're going to spend on the PSU, it might be better to just get one with a 24 pin...