I will go to Gorlitz (Germany) first and then walk to Zgorzelec (poland) by foot.
How long is it?
Is it convenient to change money on the way? How much commission would it be?
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To get from Görlitz to Zgorzelec you just have to cross a bridge. If you go up hill for a while from the old town bridge (Altstadtbrücke), which is a pedestrian bridge, you will get to a place where there are some shops and also a Kantor, which had reasonable exchange rates at least a few weeks ago and charged no commission. It's very small, just a door in a line of shops, but it has a sign so it's hard to miss.
Depending on what you want to buy/do in Zgorzelec, you have to take into account that bigger shops are far away from the border, so you should take a bicycle or try to catch a bus. Also, if you just want to buy cigarettes or alcohol, there are small shops for that just across the bridge, and I was told by people from Görlitz that they also take euros there. (But I don't know how good their exchange rates are)
https://www.kantory-plus.pl/ this is extremely close to the border and as far as I can see charges no commission. From the Gorlitz train station the border is 2km, depending on your walking speed, I would say it's 30 minutes.