You can easily order them from Digikey in unit quantities. E.g. here is an accelerometer that I've used before: https://www.digikey.co.uk/products/en?keywords=MMA8653FCR1

Most I2C interface sensors require minimal external components (often just a bypass cap), so they're are quite easy to use without a breakout board.

The one non-trivial thing that a breakout board might do for you is level shifting, if you're using a 5V microcontroller.

Those breakout boards are just for experimentation / prototyping. You typically design a proper PCB for the final project and of course you can also buy individual sensors.