Rather than buy new plastic plant pots and seedling trays - though of course, it is good to clean and reuse ones you already have - repurpose all sorts of containers!
I have my tomato 'Y Ddraig Goch' sprouting happily on the windowsill in ice cream cups I brought home and cleaned after finishing our delicious scoops of lemony ice cream, and some basil in mycelium bottle trays that came as packaging :-)