On my '93 SSEi and '92 SSE */c, the idler pulleys are fastened with a single bolt, I think it was 15mm (or maybe 13mm). To remove the pulley, turn the bolt counterclockwise just as you would normally do to remove any other bolt on anything else.
I bought replacement pulleys at the auto parts store. They're metal, but cost almost $20 each.
I tried to remove the bearings from one of the old plastic OEM pulleys, in hopes of pressing in a new #6203 bearing, but the pulley cracked as I was trying to press-out the old bearing. Close inspection revealed that the bearing is held in place by a molded lip in the plastic on both sides of the plastic pulley. I wonder how the bearings were pressed-in at the factory...I asked a friend who'* a machinist, and he didn't know how the bearings were pressed-in at the factory either.
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles