My own search for die-cast cars in all scales relevant to miniatures gaming has paid off far better in 28mm than 15mm.
I’m pickier than many other hobbyists when it comes to these things, so I only want to use cars in the 1/52 – 1/60 range for 28mm (and I wouldn’t mix 1/52 with 1/60 on the same table), with 1/56 being ideal. 1/64 is miniscule, 1/48 is a tad too big for me personally, and 1/43 is wildly outsized. Even so, with a bit of searching in toy stores I was able to find enough cars in the right scale (mostly 1/56) to suit my purposes. The primary paydirt for me has been a line of 1/56 tie-in cars for the Fast & Furious films. Just keep in mind there are many different lines of Fast & Furious die-cast cars in different scales, so don’t get the wrong ones. I’ve also found some jeeps and more mundane cars from Majorette, which is a well-established competitor with the Hot Wheels and Matchbox brands in the European toy market. Most Majorette cars are 1/64 but there are a few bigger ones in the mix. My only problem in regard to finding modern-day vehicles to populate my 28mm tables is buses, lorries and things of that sort.
Here’s a previous TWW post I’ve written on the subject: LINK
I’ve not had much luck finding 1/100 cars, or 1/87 ones. I’ve seen some available from Chinese online vendors, but for some reason they were priced much higher than most other die-cast cars in larger scales. The product descriptions/pictures were also dodgy, leaving me uncertain that I really would have received what was advertised. Furthermore there wasn’t much variety. Aside from those, the usual Hot Wheels / Matchbox / Majorette lines have some vans, pick-up trucks, campers and similar vehicles that might be in the general ballpark of 1/87 – 1/100. It’s hard to tell when you’re just looking at them on a store shelf. Scales vary wildly for anything above a minivan.
If you do start buying die-cast cars for miniatures gaming, expect it to be a bit “lossy” in that not every item you purchase turns out to meet your requirements. That’s just par for the course.