Atmega328PB -AU Chips in china with bootloader

I am starting to work on the design for a custom implementation of the atmega328pb board with some pre-wired sensors onto the board and was trying to source some parts for mass production. I noticed that I cant seem to find any chips with the bootloader pre-installed in them in alibaba. Does anyone have experience working with Chinese manufacturers in this regard? How expensive would it be to have these manufacturers install the boot loader onto each chip before soldering it onto a basic board vs having all boards send to me and then one by one loading the boot loader onto each chip. I wasnt able to find chips with the boot loader pre installed which would solve this issue, if anyone knows where I can get them cheap around 500 piece batches or so , it would be appreciated.

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Why a bootloader? Are these products intended to be programmable by the end user using a USBTTL adapter ?

Bootloader, preprogramed, what's the difference? I run a small business, send the completed boards to me and I'll bootload them and put on your code. May take a few days doing them one at a time.

I will probably just program them myself, thanks for the help everyone.