In certain cases the NCBI SRA has published datasets from submitters on a "provisional" basis, having a SRA accession but with files distributed in their native, original format. This may be done in order to accomodate a dataset from a new platform. Over time, provisional datasets will become fully archived.
If you cannot find the dataset of interest in Entrez SRA, please check Provisional SRA browser to see if you can locate it here. When the dataset has been fully loaded into the Archive, then it will no longer appear in the Provisional SRA browser.