Try calling the tire shop and see if they forgot to put it back on or something. It happens. One time I had a tire replaced on my Eagle Talon and a few days later I noticed the car was driving strangely as I drove to work. It was vibrating and the front end was feeling wobbly. I took it into another tire shop that happened to be closer than the first one, and found that the first shop had forgotten to tighten down the lug nuts! The wheel had lost three out of the five lug nuts, and I had been about to get on the freeway!
You can probably buy them at the dealer, but it might be cheaper just to replace all four of the hubcabs for better quality ones. Someone mentioned
http://autoamenity.com/ as a place to buy lookalikes.
Reminds me of when I was a kid and my parents were driving my brother and I in their new Sable (Taurus twin) station wagon. The front passenger side tire had recently blown and had to be replaced. We were stopped at a stop sign near our house, with mom driving, dad in the passenger seat, and us kids in the back. All of a sudden the hubcap on the replaced tire popped off and went bounding across the neighbor's yard. Dad immediately threw the car door open and started running as fast as he could all bent over trying to catch this hubcap while it bounced through the neighbor's azaleas. It got all the way across their yard and into the next yard before it ran out of steam. Meanwhile the rest of us were laughing so hard I thought I busted a gut. Even now I laugh just thinking about it.