Generally WTB is not allowed in this setting, the trader handles such activity, but we will look the other way in this case. Righting a 98b is a worthwhile exception; as said a 98b bolt is a $300 + component if you can find one, you could get lucky on Gun Broker or another selling site, but generally $300 or more for a nice original 98b bolt. Several that frequent this forum have loose bolts, but it is doubtful they will part with them unless a good price or trade is offered. I have found in such situations that stating your willingness to pay a ridiculous price (full retail) can elicit replies (provide a range for which you are willing to pay and it may draw out someone with a bolt they will part with... most will not offer up a rare bolt or item on the hope of a good offer, - most collectors with such "spares" often are men of means and money is little incentive, but it is your best bet...)
Yes, a 98k bolt works fine with the normal precautions (headspace, general condition of the barreled receiver and bolt, - no alterations or tampering... over the years a lot of stupid people have done lot of stupid things to rifles, decades ago these rifles were not worth much, more a novelty or shooter than collectible). Actually any German military Modell98 bolt will work, Gewehr98, 98a, 98b or 98k, also most of their counterparts in neighboring states (Czech, Polish Modell98's)