Hi there! I am not that close to Leiria, but I believe there are things nearer than what I can recommend.
However, if ou're prepared to drive out a bit, the Velharias (antiques/collectors market) in Torres Novas is a good one, and nicely set in a shady riverside area with a couple of cafés, and only a couple of minutes from the local produce market, which happens on the same days (once they re-start).
Fourth Sunday of each month, hoping they'll run it this month, fingers crossed. Check the Torres Novas Facebook page nearer the time, if interested. Put "municipiodetorresnovas" into Facebook - when I put the link here, it shows an error.
Santarém has a similar market the previous day, but it's less good, and very exposed to whatever the weather is. The good sellers tend to do TN the next day, anyway!
Also, bear in mind OLX.pt and also local groups for vendas, trocas, etc, or the Marketplace, on Facebook.
Hope you find what you need!
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