This solution is not mine, but it helped me get the last 4 cars I needed. It is so good, that I wish I had found it earlier. The credit for this solution goes to user ViperYourMother on the GameSpot Saints Row 2 forums.
Here's a trick that I've now used on 6 different hard-to-find cars:
1. Go to the district you need (Like Projects for Mag)
2. Enter and start shopping at a store (Like On Thin Ice). You don't have to buy anything, just hit Y to shop, and then back out.
3. As soon as you walk OUT of the store, you should see the yellow car icon driving around 9 times out of 10.