First things first. How much money are you looking to spend?
I'm pretty patient and willing to save however much I have to if I think it's worth it. My initial thought was that I would spend about $300. Not sure if that's naive or not.
It all depends how much you want to spend. You can get a surplus Swiss K-31 (caliber roughly the same power as a 308 Winchester) fpr less than $100. Or you can spend thousands on a custom built tack driver.