I am about to start seeds for the Community Garden at Benedictine University – the best, least expensive way to go.

Learn how to make repurposed or compostable seed pots:

Plastic

It's tough because we are learning more every day about the toxic consequences of plastic. Remember: never use plastic for hot food! But plastic pots retain moisture consistently and are therefore ideal for sowing seeds. Just don't buy them, putting new toxic plastic into the environment. The following can all be used:

the bottom of plastic milk or juice cartons;
old yogurt or cottage cheese pots;
takeaway plastic salad bowls; and
plastic salad containers.

Important steps

There are two extremely important steps you must take when reusing old plastic pots for seed sowing:

make sure the container is thoroughly clean; and
make sure you punch through drainage holes at the bottom.

Flat plastic food containers can also be kept as trays while clear plastic lids can be placed on top of the containers to act like recycled propagators, keeping seeds warm and moist during germination.

Paper

You can also recycle cardboard and newspaper for seed sowing:

egg boxes are ideal for home grown plug plants;
toilet roll tubes are particularly good for sweet peas needing room for long roots; and
newspaper can be recycled into mini pots using a Paper Potter seed pot maker.

The advantage of sowing seeds in recycled paper and cardboard is that the plants can be put straight into the ground without disturbing their roots. The paper or cardboard will naturally biodegrade.

Soil Blocking

I haven't tried soil blocking because it seems to require peat, which is environmentally damaging. But it's an attractive option because there are no wasted containers. I would be glad for comments if you are a fan. Here's one site I found on the technique: https://www.sierraflowerfarm.com/blog/2018/3/18/nitty-gritty-soil-blocking

Adapted from https://www.onegreenplanet.org/lifestyle/biodegradable-pots-for-seedlings/