It is a very broad question, that may be specified by some factors first:
What profile do you prefer? OEM, Cherry, DSA, DCS, SA?
What material do you like? PBT or ABS.
What type of legends do you like the best? double shot, sublimated, printed, lasered?
Do you want brand new key caps or you may prefer them second hand?
Some sources are QTan, Massdrop, SP, the classifieds at GH and DT. For second hand some times people offer keyboard at low price with keycaps that may be harvest, however these sets may not match all your keys in your new board.
