Digital turntables are expensive (Stanton, Numark) and unlikely to go on sale.
An analog turntable plus a Griffin iMic or any of the Firewire or USB digital audio interfaces (check and see if they have turntable input) would be more cost-effective. You can pay from $100 to $500 for a turntable -- depends whether this is a hobby or serious pro-style work and scratching.
The specialist interface is the Stanton Final Scratch 2, allows you to use your analog turntables to scratch digital audio files on your Mac. It's pricy though.