Get a track up to level S and then after your race is finished do not exit, just race again. You are still on the same calendar day. A two lap race on the Test Track at Level S is 150,000 cr. If you have the Veyron it can be done in about 3:20.
I will play this game for 5 months, guaranteed. Since I bought this game I have bought Need For Speed Shift and rented Killzone 2 from Gamefly. Neither of them have been put into my PS3 yet because I have been playing this.
Really, two days between more cars is too long for you? You do realize that it is just on a 70 day cycle, right? It works very similar to the used car lots in GT4. If you know the cycles you can spam days and get what you want when you want with very little effort. Last night I managed to run through the entire 70 day cycle on a fresh game, just to record the dealership pattern, in less than an hour. And it only took that long because I was putting the information into a spreadsheet at the same time.
Your lack of freedom is not that limited.
I mean, what day are you on and what cars are you looking for? I have a garage full of both race cars and dinky little <100hp cars. I am not having any trouble finding the cars I want. I am having more trouble affording them all at once.