The border between Germany and Poland is relatively new. The current location (Oder-Neisse grenze) was decided on after World War II.
Before the war the border had been far more eastern.

I have only visited one border town recently: Guben/Gubin, a bit south of Frankfurt/Oder.
I found border markers on both sides of the river here.
They seem to be hollow and seem to be made of wood/plastic.

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The only other visit to the German-Polish border was all in the south at the tripoint with Czechia, back in 2013.