As far as obtaining an iPhone without cellular service I can think of two options: either buying a phone out-of-contract from a carrier for full retail price, or buying a used phone off craiglist/eBay.
If you went either of those routes, you could use the phone in airplane mode with WiFi enabled.. this would allow you to use Photo Stream to sync your photos to your iMac (whenever the iPhone is on WiFi). Alternatively, you could manually sync the pictures to iPhoto by plugging in your phone to the iMac.
Is there a particular reason you want an iPhone w/o cellular service for pictures?