I'd fit them with a stainless steel bearing from VXB http://www.vxb.com/Merchant2/merchant.mvc
Plenty of different choices there, choose one to suit your budget as they are all better quality than the originals fitted at the Saris factory.
If you have the 12mm axle type of hub you'll need four 6901 double sealed bearings, that will be two for the hub and two for the freehub body.
I'd advise taking them out and double checking the sizes against your own though.
They'll take about 4-5 days to come by mail for northern europe.
Insert a 5mm allen key into the axle and undo the drive side cap, pull off the freehub body, do the same on the NDS and you'll be able to push out the axle, you may need to give it a tap with a mallet or a block of wood and a hammer to get it moving, but it's normally easy enough to get out by hand.
Get yourself a nice tool to tap the bearings out with, I use a bronze or alloy peice of 10mm bar about 30cm long, but an old screwdriver will do the job, put it down into the axle hole and push the spacer tube to one side to get on the bearing, give it a tap at 90 degree variations each time so the bearing comes out reasonably straight.
Take the spacer tube out, remembering which way the sensor magnet goes, tap the other side out in the same way, you can normally do this by getting someone to hold it in their hands to avoid damaging it, but sometimes you need to place the hub on a block of wood to get a little more shock to get it moving.
Don't beat it to death though, take it easy with the hammering.
Clean it out with a rag making sure there is no debris or burrs on the hub shoulder where the bearing sits against.
Take your new bearing, put a little bit of grease on the bearing and hub and either fit them with a bearing fitting cup which is slightly smaller than the outer diameter of the bearing, or you can use a QR spindle with the old bearings on the outside, just screw up the QR and pull them in.
I have a set of alloy cups with a shaft on them for fitting the bearings, and just tap them in, nice and straight and gently.
As a tip, you can use a socket (11/16ths is a perfect fit for this job) and a socket extension to carefully tap them in too if you don't want the hassle of making or buying a tool, just either grind the face that contacts the bearing flat so it fits nice or just be very careful to not damage the seal of the bearing, always only tap in cartridge bearings by driving on the outer of the bearing and vever on the seal or the inner of the bearing or you'll damage it and end up having to get another one.
It sounds a bit complex, but it's pretty straight forward really, just take it easy and be patient and you'll be fine.