Here's another option. Go to:
http://gt5cars.net
Click on Used Car Dealership Search.
Add the top 8 cars that currently show in your UCD list, in the proper order, of course. (Make sure that you get the name precisely right. There are many different versions of different cars and the names can be very similar.)
Add the car that you're looking for in the bottom box. (All of this is labeled.)
Click Search.
The website will tell you how many days (in-game race days) there are until the car shows up.
If you wish to advance the days in the game, there are a couple of faster methods:
Method 1: Go to the License Test. Choose the first one and let it load up the start screen for it. This counts as an in-game day. Then click Next. This counts as another in-game day. Any time you click Next or Previous, it'll count as a day, once the screen loads. You do not have to actually Start the test for this to work.
Method 2: Go to any race. Repeatedly start the race, then pause and exit the race. Click Start, to start the race again. Pause and quit. Each one of these starts counts as an in-game day.
Those are the two fastest methods that I know of. The second method seems to be quicker, but you may prefer the first one because the tests are numbered and it's easier to keep track of how many you've done by the number of the test.