How to get flat rate boxes?

Could you please explain the process of acquiring flat rate boxes for shipping purposes? Are these boxes available exclusively from certain postal services or can they be sourced from multiple providers? Additionally, what factors should one consider when selecting the appropriate size of a flat rate box to ensure cost-efficiency and protection for the items being shipped? Is there a specific process for ordering these boxes online or do they need to be purchased in person at a post office? Finally, are there any limitations or restrictions on the types of items that can be shipped using flat rate boxes?