What reference population should I select and how to access to sequence data?

Question

[reza_halabian]

Hi, I want to impute genotyped SNPs from 10k and 50k in Holstein dairy cattle up to HD genotypes and sequence level. But I am not quite sure what reference population I should select and how to access to sequence data of this reference population? I would be very grateful if some one provide me some information. Thanks in advance.