Paper won't stand up to months of outdoor culture. Neither will peat, molded fiber, coir, etc.

I suggest that you buy the plastic containers, and offer a small discount if they return them for you to use again.

That with the discount is a good idea.

I would also ask nurserymen and women in your green area what they do about pots. Nursery folk are usually pretty good about sharing information, resources, etc. It sounds like you would benefit at this point in your business by attending a nursery show. These are good places to learn what is in the market, what it will cost, what it looks like, etc. You needn't buy anything, but the information is always useful. You will probably also want to source a wholesale supplier willing to supply pots in fairly small amounts. Some of those companies will have minimums which will not likely be a good idea when you are starting up.

Self-made paper pots, certain peat pots, etc. can have a very surpisingly long shelf life. But individually sized and shaped pots can really be a headache in storage, shelf space, shelf life and handling time.

Call your local landscapers. They usually throw their pots out after they do a job.

I had some success putting the request in freecycle and at the tip they are taking the pots out I pick them up. I have as well put a request on my webpage that people bring their pots back. What I found selling plants is that people don't mind old pots they look for the plants.

Landscaping outfits are the best, if you can use the larger pots. Buy a 100 roll of snow fence, and a dozen Tbar stakes. Offer to fence off an area of their yard for pots. The fence keeps them from blowing around. You can leave some hints with partitions into sections with pallets, and start stacks of each size.

Initially come by once a week, and sort the pile. When you need a bunch of pots, bring the truck or the trailer and load up. Many places will love to cooperate with someone like this.

We got most of our pots at the dump or transfer stations.