Planner covers are not a necessary part of planning. But they’re a fun way to customize your planners and a fair amount of hype swirls around things like the twice a year release of new Hobonichi covers and drops by other cover makers.
The most popular and widely known cover makers among the planner community seem to be Hobonichi, Traveler’s Company, and Moterm. But there’s a lot more out there as well! Below I’ll share some alternatives:
Baum-Kuchen

Baum-Kuchen is a stationery retailer that offers a number of cover alternatives for different systems including a line of wallet inserts made in collaboration with The Superior Labor. The Travel for Life wallets in particular work really well in Travelers Notebooks as inserts but also as standalone covers for the Hobonichi Weeks. They usually have limited inventory so you have to check for new drops or follow them on Instagram.
For TN lovers, BK also does Travelers Notebook customizations to make your covers truly yours.
Yuruliku

Yuruliku makes something called a FLAT tool case and the A5 size fits a Hobonichi Cousin or any similar sized book with room to spare for pens, stencils, sticky notes, stickers, you name it. It’s become my primary A5 planner cover/folio.
Roterfaden

This modular system works like a Traveler’s Notebook in that you can swap in any insert you want. The difference is that the books are held in with clasps rather than elastic, and they can take several books including very chunky ones. They come in A5, A6 and B6. JetPens and Baum-kuchen carry them but if you order directly from their website you can get your cover customized.
Wanderlust by T.S.L.

This brand of The Superior Labor offers an array of beautiful leather and fabric covers in A5 and A6, and ships from Japan.
Hobonichi exclusives

Every year Hobonichi does collaborations with other companies and organizations, including the Japanese retailer Loft. These are exclusive to their partners and can only be purchased in Japan. If you’re interested in these planners or covers, you have to buy them on the secondary market (eBay or Etsy) for a significant markup, or pay a shopper in Japan to purchase it for you.
Bujo Planner
To catch one of her drops, you have to follow Marija on Instagram. She makes beautiful semi custom leather planner covers in all sizes.

Other custom leather cover makers:
Midori

Midori makes simple clear and goat leather covers for A5 notebooks.
Etsy sellers
There are also covers of all styles on Etsy and other marketplaces!