I use empty yogurt containers, the small individual ones are great for individual pepper/tomato/eggplant seedlings. Just make sure you get the ones that are smaller at the bottom than the top (I think yoplait ones are the reverse) so the seedling is easier to remove when you transplant.
I’ve use the bigger yogurt/sour cream/ricotta containers for larger seedlings.
I also save the large styrofoam drink cups from drive thrus. Hubby and I get our family to stash those in a box in the pantry and we collect them when we visit. Those don’t usually survive the planting season but they were free and they were going to be thrown away anyways. Ditto the large el cheapo plastic cups from the drive thru. Those sometimes survive several planting seasons.
Even if you don’t use any of these things you might have relatives/friends/coworkers who do and might be persuaded to wash them out and keep them for you in return for produce from the garden. Or for decent seedlings for their own vegetable patches.
I’m not an eco wacko but I am el cheapo.
The last time the engineer was here before I started seeds, he couldn't figure out why I had empty bakery clear plastic containers from the grocery stacked in a corner of the kitchen counter. Told him those were to be trays to sit paper cups in that would have seeds in them. He said he knew I must have a reason. He is not a prepper. He thinks the only danger the country faces is global warming. I still let him stay here when he is going to a conference rather than hitting him over the head and kicking him out for being a liberal.
He is as smart as Einstein was, and he (engineer) was the subject of a Ph.D. study done on him when he was six or seven. He was tested for several weeks during that. I wonder if Einstein would think global warming was our worst danger as the engineer does?