I have been using some industrial parts trays that have lasted me about 15 years, often sitting outside for half a year in the sun. But I am growing more seedlings now so have been buying these Sterilite clear plastic pans that are about 16 inches by 23 inches with sides about four inches high in Target, right now they are on sale for $3.49 each, including a blue plastic lid. Trouble is this kind of plastic deteriorates in sunlight due to UV damage if left outside for several months and start breaking up when you lift them.
I see some people recommending a brand called Perma-Nest that are supposed to last several years, but they are very expensive, and I can't find any local sellers and the online sellers want around $7.50 apiece for an 11 inch by 22 inch by 2.5 inch tray plus shipping at the cheapest place I can find. They are made by Growers Supply, but they won't sell retail.
I also look for metal baking pans or plastic containers in Goodwill or other thrift shops that I can use.

Cardboard beer flats or cardboard boxes with the sides cut down work great for transporting. They are what we use to haul plants to the plant sales.

Perma-Nest trays are expensive compared to regular trays but they do last forever. Regular seedling flats are cheap enough that you can double them up for extra strength and you can water the plants in them too - unlike the cardboard flats. ;)

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Dave

I grow a few hundred plants and what I've used is the plastic shuttle trays that retail nurseries get their containers of 3" to 4" round seedlings packed in. As the plants sell they discard these trays. Last year I went to different big box stores (Lowe's, Menards,Home Depot, etc.) and pulled dozens of them off the carts they used to stack them on before they toss them in the garbage. I chose the ones that were the right size for the cups and each tray holds between 8 to 18 plants each, depending on the manufacturer.

So start gathering them when your area stores start selling 3" annuals and you'll have free shuttle trays to use each season!

I use plastic "under the bed" sweater boxes. The high sides provide stability for the cups and you can fit alot into some of the longer ones. Granted, they are not the toughest things and can't be used in the sun for long periods (months and months), but they are easy to find and pretty cheap. The lids also make good "coasters" for pots in the house.

I stop by landscaping crews in flowers planting time and get their empty flats, 3-4" pots that they would otherwise discard. I have plenty of those flats, six packs and bunch of round and square small pot.
Cardboard boxes (from liquer stores and elsewher) is also a good idea.
For very small seedings I also buy plastic cold beverage cups from dollar stores. They take less room and are individual and easy to emplty and transplant root-sensative seedlings. But you have to make holes at the bottom.