Google search images pagan altars, wicca altars for inspiration. There is a redditaltars lots of pictures, Youtube videos of altars and witchy hauls for altars.
A walk in nature you can find items or a seaside walk: shells, sand, stones.
Home decor shops: candles, candleholders, bowls, incense burner, oil burner, placemats as altar cloth. A good online shop is AzureGreen.