There are third party knockoffs you can buy for between $20 and $50 or so that fit the 38/40mm. I bought a $20 one off Amazon for my 38mm S0. It looked and felt great for about a year or so (rotating it with a couple other bands). Eventually the leather like material (it definitely wasn’t real leather) began to peel and not look so good. I suppose it lasted long enough for $20, but I would gladly pay the Apple premium for a genuine leather loop. It boggles my mind that they never made one for the 38/40mm case size.
I may order another knockoff. I really like the loop type bands. Lately I’ve been wearing the sport loop that came with my 40mm S4. I might just get a second sport loop in another color, but the leather loop strikes a nice balance in terms of style. It’s more stylish than a sport loop, but not as formal as a Milanese loop. I think the leather loop is the most iconic band for the Apple Watch, so again it just puzzles me that Apple wouldn’t make it in both sizes.