To clarify, are you asking about minor allele frequency or mutation annotation format?
If the latter, MAF (format) files are part of the level 2 mutation data for each cancer type.
If the former, some MAF (format) files contain readcounts and variant allele frequency information, but it's an optional appended field, IIRC. For those MAF files that lack readcount data, you'd have to pull down the bams and generate that information yourself. You might also look in the VCF files, which I think contain that information.
If you're looking for the global minor allele frequency, you'll need to cross-reference sites from the MAFs or VCFs with population data from dbSNP, 1000 genomes project, or exome variant server.