Cancer sequencing data as fastq are hosted e.g. at TCGA/ICGC and dbGaP. databases. You need to read papers that created them and find the respective accessions. Be aware though that these are always access-restricted so your PI must apply for access via your institution, then access might be granted upon reasonable request. This is not optional and there is most likely no way around this. This is going to take weeks though as the application gets formally reviewed, maybe rejected first asking for edits etc, which is why you should always work with an experienced supervisor who has knows how things are going in the field. From your previous question I take that you are doing an undergraduate project. If you have no background so far, and no data at hand so far, this is likely to be an extremely cumbersome process. Get a supervisor!