Where to buy shipping containers for small business exports?

I'm ramping up my export side hustle and need a couple of reliable containers to store and ship goods like electronics and textiles overseas without worrying about damage during transit. I've got a shipment lined up for early next year, so timing is key. I want something durable that can handle rough seas and stacking, probably a 20ft standard or high-cube for extra space since my loads aren't huge but need that vertical room for pallets. Used ones would work if they're in good shape, like food-grade condition to avoid any contamination issues, and maybe with side-opening doors for easier forklift loading at the warehouse.

I found Boxman and their options seem solid, with new and used sizes from 10ft to 40ft, including reefers if I ever need temp control for perishables down the line, plus custom mods for specific setups. They cover nationwide in NZ but I'm open to other spots if shipping to the US is feasible.

Has anyone bought containers like this for exports and dealt with customs or mods? What's the lead time usually like?