The Collaborative International Dictionary of English v.0.48:

Supportable \Sup*port"a*ble\, a. [Cf. F. supportable.]
   Capable of being supported, maintained, or endured;
   endurable. -- Sup*port"a*ble*ness, n. -- Sup*port"a*bly,
   adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

supportable
    adj 1: capable of being borne though unpleasant; "sufferable
           punishment" [syn: bearable, endurable, sufferable,
           supportable]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "supportable":
   acceptable, apodictic, attestable, authenticatable, bearable,
   believable, checkable, confirmable, defensible, demonstrable,
   demonstratable, endurable, evincible, livable, provable,
   substantiatable, sufferable, sustainable, tenable, tolerable,
   validatable, verifiable