What is the Biblical support for Purgatory? Exactly as the title asks; where do Catholics get their Scriptural support for the idea of Purgatory? To be more specific, how I think of Purgatory (which may be incorrect) is that it's a temporary place of suffering before one goes on to Heaven.
From what I understand, the idea behind Purgatory is that it is a place or state where sinful believers go before they are admitted to Heaven.  It is believed that it is possible for the living to pray for and atone for the sins of the dead so they may be admitted into Heaven.

This belief is mostly based on 2 Maccabees 39-46, which is a part of the Catholic bible.  (This book is considered apocrypha for many denominations.  See What is the origin of the Catholic apocrypha?)

(Italics were added by me in the following)


  2 Maccabees 12:39-46 (DRA)
  
  39 And the day following Judas came with his company, to take away the
  bodies of them that were slain, and to bury them with their kinsmen,
  in the sepulchres of their fathers.
  
  40 And they found under the coats of the slain some of the donaries
  of the idols of Jamnia, which the law forbiddeth to the Jews: so that 
  all plainly saw, that for this cause they were slain.
  
  41 Then they all blessed the just judgment of the Lord, who had
  discovered the things that were hidden.
  
  42 And so betaking themselves to prayers, they besought him, that
  the sin which had been committed might be forgotten. But the most
  valiant Judas exhorted the people to keep themselves from sin,
  forasmuch as they saw before their eyes what had happened, because of
  the sins of those that were slain.
  
  43 And making a gathering, he sent twelve thousand drachms of silver to
  Jerusalem for sacrifice to be offered for the sins of the dead,
  thinking well and religiously concerning the resurrection,
  
  44 (For if he had not hoped that they that were slain should rise 
  again, it would have seemed superfluous and vain to pray for the 
  dead,)
  
  45 And because he considered that they who had fallen asleep with
  godliness, had great grace laid up for them.
  
  46 It is therefore a holy and wholesome thought to pray for the
  dead, that they may be loosed from sins.