For RENIX, I know the idler pully takes a Timken #203FF bearing - I'd imagine later models would be similar (having the part number from one vendor should give you the ability to cross-index with whoever is local to you.)
Installation is easy - I've done it in the space of about two minutes with a vice and all parts at room temperature. There isn't much interference fit there, so heating/cooling isn't really necessary. I do suggest lubricating the outer shell of the bearing when you change it - I used never-seez, but any quality grease would work as well (makes it easier to install, and makes it easier to remove later.)
You will need a socket that is just about the same size as the outside of the bearing or a bit smaller (or a small section of metal rod, or a short bit of pipe) to get the thing started. You'll also want a fairly large socket to receive the old bearing and give you something to "push into" so you have room to get the bearing out.
Yes, replacement assemblies can be had over here - but it was two minutes' extra work to do, and $3 for the bearing vice $15 for the pully. I put that extra $12 to better use elsewhere. (What can I say? I'm a Scot!)