Three places I go if I'm trying to track down scores (if IMSLP doesn't have them):
https://www.boosey.com/cr/perusals/You'll need to create an account here, but they have scores of numerous contemporary composers, as well as non-PD scores of Bernstein, Britten, Copland, Stravinsky, and others.
https://issuu.com/Issuu is a digital publishing platform, but they have loads of classical music scores as well. A way I find a score I'm looking for is to enter into Google "[composer name] issuu scoresondemand". I've found a lot of scores of Malcolm Arnold and Vagn Holmboe, among many others.
https://www.nkoda.com/Nkoda is a subscription-based service that's specifically devoted to sheet music. They do offer a 7-day free trial. The subscription is a bit pricey but worth if you find what you're looking for. They have a lot of composers that I've been unable to find anywhere else.