We turned to buying stuff from the 'school pool' aka "let's sell the lost property because the heap is getting too big". This is for school uniform items.
Op-shops. Hand-me-downs from other kids. If it had holes, I would fix them. We still do this - frayed jeans with holes are in fashion now anyway - even buying new jeans for difficult child 3 is difficult because the brand new ones are sold pre-frayed. Crazy. And when difficult child 3 (previously with difficult child 1) gets holes in his jeans, I patch the holes - with denim cut from jeans now too far gone for use. I sew the patches on with a decorative stitch, maybe add a safety pin and hey presto! Fashion!
Also, I labelled everything by writing inside the collar with permanent marker.
What also worked - don't know if you can do this - was sending him back to fetch what he had left behind. So if he came home without his lunchbox or without his coat, I took him back for it. We also would make do for the next day, sending him out of uniform which meant teachers kept on asking him why he was out of uniform, and when he said, "I've lost my coat/hat/sweater," there were more adults telling him to go find it.
If all else fails, set aside a certain $ amount based on what you are already spending. Tell difficult child that this is the allowance for a certain period of time. If you spend more than that, he has to work off the amount extra. And if you spend less than that (because he takes more care) then the balance of what is left can be spent on something non-clothing that difficult child wants.
If you do this, keep a running tally on a sheet stuck to the wall - write down everything lost, the date and where (you think) plus how much you spent to replace it. Once you spend money to replace the item, finding the item will not lead to him getting credited with that amount back - if he lost it for long enough to get replaced, he's done the dough.
Use your judgement, keep it as open and transparent as you can.
Also, work out how much you would spend in a month. Depending on how fast he loses stuff, you can make the time period longer or shorter. But shorter is best, because for every successful interval, he gets a better chance of winning sometimes.
Good luck!
Marg