"By putting biome-specific flora and other items into a bee hotel, you can create a small environment that will attract solitary bees!"
Flower seeds are items that allow you to grow new flowers. They are usually got through extracting Frames that have been filled with bees - the flowers near the hive at the time the frame was filled will be used to determine what seeds are gained, sometimes causing new flower seeds to form.
Seeds can also be gained by digging up seedlings with a Spade. Using a Watering Can on a newly planted seed will cause it to immediately grow into a flower.
Each flower has it's own seed packet as show here: