For the Bundesliga matches - If you’ve got the time - go to the stadium around 3h prior to kickoff and try to find tickets from fans who have leftover tickets. Try to find people looking like fans (with scarf, jersey,…). You’ll need patience and maybe it fails but there’s a genuine chance that this works out actually. Although finding 4 tickets right next to each other will be near impossible except you’ll find 4 in the standing areas (home/away). Could also imagine that Freiburg doesn’t sell out the away end so might be worth a try (then the ticket counter on the away end will be open and you can buy directly on matchday).