We don't sell replacements of the removable insert that comes stock with our safety boots. Here are our recommendations from other brands.
Xero Shoes sells a very breathable insert, which one review notes is 3mm. If you feel hot in your boots, this is a good option. They use athletic sizing, so order their size 11 for your size 10 Carets boots. $5.95.
Vivobarefoot sells cork inserts if you run hot, or thermal inserts if you run cold, also 3mm. $12-14.
Lems sells much thicker inserts, 4.5mm and 5.5mm, if you want more cushioning. But the extra thickness could reduce toe box room, as well as the efficacy of the steel toe's Impact and Compression resistance. $10-15.
NorthSole seems to get high praise from the barefoot community, but it is more expensive than the others at $25.97.
For extra warmth, check out this article on insoles from Anya's Reviews for a list of wool insoles.