Newegg.com has the Obihai OBI100 VoIP Telephone Adapter with Google Voice & SIP for $29.99 after -$8 off coupon EMCWPGW227 (exp 11/6).

Features:
Google Voice Officially Supported Device
Easy to Set-Up Using OBiTALK.com
Free Domestic and Super Low-Cost International Calling
Works with Up to Two (2) VoIP Services