What I used to do was grind the Ferrari seasonal, do that 5 times, check UCD, repeat
Great way to make money and cycle UCD at the same time
Or a faster way to cycle the UCD is start exit a race 5 times
I'm on day 6000+... so i've done a lot of UCD refreshing. Just be patient, it can take up to a month or more to find a car you're looking for..