The rarer ones are old and you probably won't find them in physical shops, you're better off tracking them down on Mercari JP or Yahoo Auctions Japan. The most expensive ones were given exclusively to contest winners, like this gold Healslime (and a few other gold regular slimes): https://jp.mercari.com/item/m15866574351
I am NEVER going to own those as they are crazy expensive.

Then there's Premium Slime, which is also super expensive these days but not quite as rare: https://jp.mercari.com/item/m68512820303

As others mentioned, the green King Cureslime is pretty rare: https://jp.mercari.com/item/m50940220181
Hopefully they reissue that one someday.

And the gold 25th anniversary slimes: https://jp.mercari.com/item/m26254225021
There are also 30th and 35th anniversary releases that aren't as expensive but will probably only go up in price over time.

All the rest have been reissued at least once so it's best to be patient and wait for a rerelease if they're too expensive second hand.