I am not aware of any requirement to have a death certificate in hand before appointing someone to replace a deceased member of the board.  Most boards just act on their understanding of the situation.  Most bylaws or state nonprofit corporation laws permit the remaining board members, even if less than a quorum, to proceed to fill vacancies on the board when there are no voting members to act.  If you have no voting members, I suggest that the remaining director just proceed to appoint the replacements.  The deceased are not likely to return to protest.

If you really feel the need for a death certificate, they are normally available to anyone from the bureau of vital statistics in the locality in which they died.  Call the bureau to determine what you need to do to obtain copies.

You also mention that you used to have three directors “required by the government for our 501(c)(3).”  Three directors are not required by the Internal Revenue Service to qualify for (c)(3) status.  Three directors are often required by state nonprofit corporation law, but some states permit fewer, which is acceptable to the IRS.