To stop the exodus of players to France and England, Thomas says that the Welsh Rugby Union and regions have to act.
"I'm a firm believer that if you want to play for Wales you play in Wales," he said.
He repeated a call for the WRU to re-instate the Professional Rugby Game Board to look at the matter.
The PRGB, comprising the four regions, the WRU and an independent chairman, was set up following a professional review of the game in Wales.
It has met only once since being formed in December 2012.
He believes that a policy not to pick players outside Wales would make stars like Jamie Roberts - who is leaving the Blues at the end of this season - think twice about moving.