You can get a Sonnet on eBay. They run around $50-70 or so. But if you want to save $40-70 and still have the equivalent look at this thread.
I followed harrymatic's guide and he saved me literally $40-70. My card is bootable and performs just fine.
You're looking for a PC card with a Silicon Image SIL3112 controller chip. What I did was a Google search for: ebay.com "SIL3112+SATA+card"
I got a PC card with that chipset for $10.76. Flashing it was dead easy and other than getting the correct data cables to the correct drive once hooking it all up it was smooth.
That process looks very promising. I'm tempted to do it for kicks for one of my old Power Macs. Neat.