I'm afraid the problem is modern classicist experiments with ragtime are generally still covered by existing copyrights (although many of them are still in print and available). I noted that IMSLP has Debussy's "Le Petite Negre", but couldn't find "Golliwog's Cakewalk." The wonderful "General Lavine" is contained in one of the preludes collections. Stravinsky's Piano Rag Music i believe is still under copyright, as are Milhaud's syncopated pieces such as "Saudades do Brasil." The final movement of Hindemith's "1922 Piano Suite" also features ragtime, as well as Satie's "Ragtime Parade." These should for the most part be obtainable as newly published editions.