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. We use dress sizing vs their athletic sizing, so if you're a size 10 Carets, order their size 11 insert. $5.95.
Vivobarefoot sells cork inserts if you run hot, or thermal inserts if you run cold, also 3mm. $12-14. They use the same sizing as us, so order the same size.
Lems sells much thicker inserts, 4.5mm and 5.5mm, if you want more cushioning. But the extra thickness could reduce toe box room. This also reduces the steel toe's Impact and Compression protection, because any dents incurred would be closer to the top of your feet. We use dress sizing vs their athletic sizing, so if you're a size 10 Carets, order their size 11 insert. $5.95. $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.