I would try contacting klingspor; they're an american company that's always ranked with the best abrasives out there but small enough to offer odd sizes. Give them a call and see what they have to say. I haven't dealt with them in a while but they seem like good people and I'm still enjoying there sanding disc's and belts to this day. .4" (3/8) x 13" (10mmx330mm ) belts maybe hard to find as it's more of a across the pacific size but contact the ones I mentioned.
EDIT: These are really good belts Norton Bluefire in your size:
https://www.zoro.com/search?q=3%2F8%22%20x%2013%22%20%20belt&categoryl1=Abrasives%20%26%20Polishers
Good sandpaper makes a massive difference over the cheap stuff, you end up paying more in the long run buying the cheap stuff. Every time I try going cheap its instantly regrettable. I've had custom sized belts made for me by good companies and then try the cheaper ones and those are shoved into a dark corner for emergency use but hoping never having to use them. Be careful of the ones you find on Amazon, they look good but then read the reviews. I don't get how they get high ratings with so many bad reviews.
Classic example is Harbor Freight sandpaper, looks good and priced very reasonable but pure junk especially their wet/dry sheets. Their 1000 grit is more like 220 grit and doesn't last long, I'm not joking. I do a lot of lapping and blown away how rough their fine stuff is and how quickly it loads up. I like using Norton, Klingspor, Mirka and 3M sheets and belts whenever I can.
There's quite a few companies that will sell you made to size belts made to your size reasonably if you order enough (like 15+).
I would also contact Tru-Grit, they sell zillions of sandpaper and belt sanders to the knife making community and its there speciality and priced very well. Call them, they're considered one of the very best suppliers of sanding belts.
https://trugrit.com/product-category/abrasives/abrasive-belts/
https://www.klingspor.com/InterNat-Lead/ContactWW.asp
