The character of Randy Warsaw is both named and based on modern artist Andy Warhol. Though he physically resembles Andy Warhol, he is actually an amalgamation of many famous artists, including Henricus Antonius "Han" van Meegeren, Banksy, and Jasper Johns. They also had associates who forged their own paintings, and the graffiti element is clearly based on Banksy.
The band "Sunday Around Noonish" and its members Butch Firbanks and Eeko are a homage/parody of the band The Velvet Underground, and its members Lou Reed and Nico.
As noted by someone else, the band "Sunday Around Noonish" are a riff on the Velvet Underground, but their name is also a parody of that group's classic song 'Sunday Morning'; and 'SAN' band member 'Eeko' is a parody of the VU's sometime singer Nico, for whom 'Sunday Morning' was written.