If they truly need to be replaced get the correct ones from your local shop or from a reputable online bike shop or direct from the manufacturer. Sadly the good folks at Tacx (or maybe Garmin did it) decided to get rid of their aftermarket pulley wheels which were fantastic and I have them on an old Dura-Ace 7400 Derailleur and would have bought way more had I known at the time they were not going to be available anymore.

I wouldn't use 10t unless the derailleur was built for them. More teeth is always better (to a degree) Not to say that one should go out and get those OSPW or Kolossos cages but they do make a ton of sense. Better chain wrap, more durability because you have more teeth supporting the load and could also in theory run larger bearing balls which will last longer. I am not saying you need ceramics or super fancy stuff but I think if Shimano made a high quality derailleur with larger pulley wheels (like a Deore or higher end version of the crappy Altus) that wasn't anything special just normal delrin and their normal bearings, they would have something even greater.