You might want to check a local garden center for the pea gravel. They probably have large bags of it with the mulches or building supplies.
Putting gravel over seeds will cause them to not be able to sprout. The gravel will cover the seeds and the little seedlings and they will not be large enough to cause it to move aside.
Do not apply the gravel until the plants are transplanted into the area at 6-8 weeks or after they are that age in the garden.