For what is worth, I seen a cracking recommendation on here to use 4 x 1.5kg girly dumbell things. They fit perfectly into an ammo pouch. And 4 plus a water bottle equals exactly 21.5 lb. That's what my lad used in RT. He started off using socks filled with rocks. Although 21 lb ended up being 30 lb.
Regardless of circumstances, one is unlikely to get any sympathy from a Training Team. It's a personal admin issue: each individual is responsible for having the correct kit at the appropriate time and place.
But kit can go adrift or—even worse—get pinched. In which case the individual has to deploy resourcefulness to overcome the problem.
On the topic of webbing weights as referenced by @cc1 I made my own: gravel bagged up inside old socks, moulded in a mess tin and wrapped with a mile of tape. Ugly, but effective and cheap.
Why waste one's pay on buying kit when it can be invested in beer at the weekend?