Where can I find the genotype frequencies for the SNP's that are tested at 23andMe, Ancestry and FTDNA/MyHeritage?
gnomad is probably the most comprehensive allele frequency database that exists at the moment.
Depending on how many variants you need to query, you can either manually input them into the search bar, or if you have a lot more, then you can also directly download the vcfs under the downloads tab.
The default for V2 gnomad set is on the gr37 build.