When to shop around for mortgages? Before or after making offers

[Victor Vela]

I am wondering what would be the best scenario to get the best rates in a mortgage:

Either contact as many banks as possible before starting to make offers or contact them after your offer has been accepted and you have a specific properties and the bank/lender can lock their rates for some time.

I worry that with few days and many other deadlines shopping around after might not be enough.

Would you also recommend contacting a mortgage broker?

Thank you in advance!