"I downloaded the file, but Stella says it is corrupted." Solution: You likely downloaded a bad dump from a random site. Use the verified No-Intro checksum. The correct SHA-1 for Video Olympics (USA) is 9c5c4c6f8a1e... (Check No-Intro database for accuracy).
While there is no official "Pong" cartridge for the Atari 2600, several homebrew versions and official equivalents are available for download. Official Atari 2600 Equivalent The original Atari 2600 game (1977) contains "Pong" and over 50 of its variants. pong rom atari 2600 link