Obviously JL cars are highly sought after, especially the Playing Mantis era cars. Which I can sometimes find if I look hard enough. Results may/will vary, depending on where you live. I live in [the greatest state ever] Michigan, USA. So this applies mainly to the USA.
My first suggestion is the flea market. It's where multiple people trying to sell either garage sale items, items they horded over the years and need to get rid of, or people who actually track down items to sell (like an actual store). I haven't personally found any JL cars at flea markets, but I do know it's possible.
The next place is antique stores. The name is a lie, they don't only sell antiques. There are multiple vendors who sell whatever they what.
Speaking on that, both flea markets and antique stores are pretty mucj near identical in how they are set up. Both have multiple vendor booths that sell whatever they fancy. You need to look for the following types of booths; diecast toys, hot wheels only booths, toys in general, booths that deal with cars...get the picture? There are also stranglers, who are in a random booth. You never know. Just do what I do, go down each and every aisle and end cap, looking at everything. You'd be surprised at what you find.
Another great avenue are collectable stores, as they sell collectables (obviously). And if you act nicely, they may track a JL car down for you. There is a collectable store near me that if I ask, he'll try to track the item down.
The next thing I wish to tell you guys is price. The collectable store guy is pretty reasonable, so he can't be included in the portion. I'm talking about the flea market and antique store prices.
In order to rent a booth, you must pay money. There's also the cost of the booth's renter original price of the item (when the bought it) and any additional money that can be counted as income for them (if the item is $3, and they sell it at $5, they make $2). So prices can vary a bit, as each seller sets his or her own price! You might find a JL car for $6 at one booth, and $8 at another! So search around! I actually have a antique store, 5 miles away from my house, with JL cars, but I never buy them there. Why? Because they want $10 for each one.
Lastly, with all three places I suggested, there's something that must be said. Their stock changes. If I only have 3 pt cruiser JL cars, once I'm out, I'm out. And if they want people to return, they have to get new stock. The collectable store guy gets all his stock from Craigslist, seriously. So you need to check every couple of weeks to see what is new. I wouldn't suggest every week, as the stock usually doesn't move that fast, imo.
Here is proof, that I'm not crazy. You never know what you may find, :).
Happy Hunting!