I'm a diecast collector and there are plenty of good M3 diecast models out there. Your best source is to find one off of ebay. There are both budget (Motormax) and high-end (Kyosho) diecast makers and unfortunately high-end M3's are highly desirable in the diecast community, starting up from $80 upwards of $200 so you may be better off with a more affordable Motormax model coming in at $30-40 to your door. But of course don't expect high-end details on a budget model if that's what you're looking for.
The cheapest high-end version is this one below, it's not that desirable but you'll still end up spending almost $100 on it. There are road versions in AW, IR, IB, etc. but command serious prices.