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It is still possible to obtain a visa at the border.  The cost is $50, £30, or €40.  Any of those currencies are generally accepted, but there have been occasions when immigration officers insist on US Dollars.  Visas are not required for Ethiopian citizens.

By the way, have you checked out the safety aspect of coming to Kenya by bus?  There have often been cross border clashes and the road from Moyale to Marsabit has long been known as a danger zone.  I believe that things are better since the road was finished though, but still worth checking out.  Many Kenyans I know wouldn't drive that section of road, but perhaps this has more to do with its past reputation.

It has been a few years since I crossed the border at Moyale, but as far as I know this is one of the border posts were visa on arrival are NOT available. This was always a rather unruly area, and it is still not completely safe. An Ethiopian rebel faction is active there, and the area rumbles a bit. You could be finding yourself with an armed escort on the Kenyan side.
It is however a great trip, and the people are very friendly.

Why do you not get a visa online?

To the best of my knowledge, the visa on arrival is available at all border points, including Moyale.  Yes, of course the e-visa is also a realistic option, but requires scanning and uploading of a photo, travel itinerary, return travel tickets and suchlike.