Asking such general question, without specifying the hub model is not enough. If you want to receive a proper quick answer without additional questions from Novatec, you need to specify which model of hub you have, for example A291SB, D772SB or F482SB etc.
The reason is simple: Novatec hubs use different bearing sizes depending on model name, so there’s not just one size/type of bearings for all hubs.
It is given by construction of hub and mainly by the purposes of hub. It’s obvious that downhill hub must be built on stronger and more robust bearings than light road hub.

So, once you specify your hub model it is a matter of minutes for Novatec staff to check with technical drawing of that concrete hub and see which bearings are fitted in there. Then this information will be passed on to you so you can get corect bearings for your hub.
Novatec uses standard sealed industrial bearings (NBK or EZO) so you should be able to find replacement bearings easily in your local hardware store. Or check the availability with Novatec EU as they have some bearings in stock. 

In order to become more independent from Novatec and to be able to find out which bearings you need for your hub, please check the pdf file below in Documents section. It’s called Novate Hubs Guide 2015. There is a list of hubs including bearing size specified for each hub in there.
This list requires an updated as you won’t be able to find some newer hubs in there, but for most cases it's useful.
This website also provides info about bearings sizes for each listed hub so you should check the hub section.
If you can’t find your hub in here don’t hesitate to contact EU office and ask them for this info.
Or check the hub section in this site where all listed hubs have also their bearings size mentioned there.

Please note that bearings used in Novatec wheels are listed in the back of wheels catalogue 2017 (in 2 pages with wheels spec) so check that out in Useful links section.

Another important information related to bearings and their replacement is this: do not try to exchange the bearings in the cassette body, it is not worth it. It is a common procedure to replace the entire cassette body as this is usually quite damaged and bitten up by the cassette at the time when bearing replacement is needed. So focus just on the bearings inside the hub shell and deal with them only, do not bother with bearings in the cassette body.

And don’t forget: keep your bearings well maintained (clean and lubricated) so they won’t let you down when needed the most.