If your tensioner is good you can just replace the pulleys. I would also replace the idler pulleys at the same time. Most autoparts stores will have the serpintine belt tensioner pulley, however the others may have to be ordered. You can also contact Gene at gmpartshouse ( a forum sponsor) and he can get you all 4 pulleys. for serpintine and ac belts.
Here's a post by forum member Evil-Twin.
You have two belt systems on your car. One is the drive belt system, the other is the air conditioner belt system.
The drive belt system consists of the serpentine belt,the power steering pump, alternator/gen, water pump, harmonic balancer, idler pulley,belt auto tensioner pulley.
The AC belt system consists of the compressor, harmonic balancer,idler pulley, belt auto tensioner pulley.
First thing you need to do is isolate which system is squeaking. this can be done by releasing tension on the drive belt tensioner and removing the serp belt. This is the outboard belt of the two belt systems..if the squeak goes away, then it is the belt or a component of the drive system.
If the squeak is still there, then it is a component of the ac belt system.
to verify this release the belt tension from the tensioner and remove the belt. if the squeak goes away, then it is a component of the ac belt system..
A common problem is the belt tensioner on either system.. but verifying which system is at fault is the proper procedure.
You could replace both belts, but you may still have the squeak. So if you want to find the culprit, isolate the system, then spray the belt of the offending system and see if the squeak goes away. Try to stay away from the idler and tensioner while spaying..
I find that belt dressing may be good to identify a belt problem, but it is a temporary fix at best..
The real problem has been the tensioner and the idler.. both of these components have a sealed bearing which has been know to dry up or redirect the grease away from the bearing, thus causing the squeak.
If you find that a belt change does not correct the problem, then a change of both the idler and tensioner pulley is in order. If you just change the idler or the tensioner alone, it will cause the other component to go bad in a month or so. This has been the case with many members..There was a change made to the dusts seals to help correct this problem in newer C5's.
Lots of tensioner and idler pulleys have been identified as a problem.. and it can effect either system. One is not more prevalent than the other. So find out which system is squeaking first, in the drive belt system, it could also be the bearing of the alternator, power steering pump, or water pump too, but these are less probable.
I hope this makes things a little clearer