The iMac has a line-in. It is a three-in-one contact: line-in, mic-in (for headsets) and optical digital in.
No need for an USB-turntable. You could get a turntable with a built-in phono pre-amplifier, but without the USB, and connect to the line-in in your iMac and use the GarageBand software that came with your computer to record the LP's and transfer them to iTunes and from there to iPod and the like. You only need the correct cable.
rca-phono pair in one end, and a mini T plug in the other end (for the iMac line in), you can get them at an Apple retailer.
If you want to buy new there's plenty of options.. Pro-Ject, Dual, Thorens, Denon - all have decent entry level turntables and some are also offered with built-in phono pre-amp. If you want the easiest thing, just plug-in and go and pressing play, here's the Denon.. It's a good sounding automatic for little money and decent quality of components (remember it is a budget entry level turntable so it is not world class). http://www.lpgear.com/product/DENDP0300F.html
Or you could buy a 2nd hand turntable for similar money but with higher sound quality, something from the likes of Thorens for example that has offered many options during the years, and a separate phono pre-amp. Usually a nice 2nd hand turntable only needs a new belt, a new cartridge and some adjustments to go up and spinning.