Bearing spec in idler pulleys

[spongebob]

I´m going to install an UDP and a low temp thermostat before the start of this season.Guess that it might be a good idea to change the idler pulleys also but to keep my limited Boxster budget under control I have an idea of only changing the bearings in the pulleys and keep the plastic parts.
I read something regarding this long time ago but I can´t find the info again in any of the Boxster forums.
Anyone here who has done this budget repair and know what bearings that should be used?
Right now Im only sourcing parts. As my car is 600km away and I plan to do all upgradings when I pick it up I cant just pull one idler pulley off and look into it