Man, the CW superhero crossovers were legendary! The 'Flash' and 'Arrow' crossover with 'Supergirl' is spread across a few episodes, but the main event is the 'Invasion!' arc during the 2016 crossover. You can catch it on Netflix if it's still available in your region—they used to have all the CW shows. Otherwise, check out HBO Max, which absorbed a lot of DC content. The crossovers aren’t just one-off episodes; they weave into the seasonal plots, so you’ll want to watch 'Supergirl' S2E8 ('Medusa'), 'The Flash' S3E8 ('Invasion!'), and 'Arrow' S5E8 ('Invasion!') for the full experience.
If you’re outside the U.S., things get trickier. Some regions have these on Amazon Prime under the CW’s licensing, or you might need a VPN. Physical copies are an option too—the crossover arcs sometimes get bundled in DVD sets. Honestly, tracking down these episodes feels like a mini-adventure itself, but it’s worth it for that classic superhero team-up vibe.