Well if money is not an option, this is the only place I've seen that still sells GBA flash carts with RTC in them.
http://www.ic2005.com/shop/home.php?cat=4
Kind of pricey though, and I have no idea how good they are or if they're well supported. You should check the buy/sell part of this...
I picked up an EZ Flash III recently as well. I managed to get it working using my iMac and virtualbox to run XP. All I had to do was set up virtualbox to see the USB port and the writing device, and it's worked perfectly.