At a restaurant supply store.

Marble-topped tables are charming, but they will etch and stain. There is no "special" table with a marble top that will "never show a mark" as you said. In a restaurant, there is so much else to look at, and the table is usually crowded with place settings, a candle, etc., and the lighting is usually not bright and you are busy thinking about the food, that you don't notice what the marble looks like. Or, some people feel stained and etched marble table is like a badge of honor showing a well-used material that people enjoy eating their food on.

Wood tables - if they are real - are also going to show wear in restaurants. Either the wood tables in restaurants that you are referring to remain pristine because they are not actually wood, or they are constantly covered in table cloths, or they are covered in a gymnasium-floor-type finish. Even with that finish, they still will get scarred with heavy use. Look more closely at the tables the next time you notice them. You might also ask the restaurant manager for the source of his/her tables if you like them.

Wood tables have what is called a "bar finish" which is essentially like the finish that is on wood floors. My mother had this done for her dining room table that she had refinished. It's not completely indestructible as she used trivets for hot stuff and placemats typically but it received normal use and looked fine for many years.

My friend had the leftover slab of her Carrarra marble used for a kitchen table. It was in the style of the Eero Saarinen Tulip Table. Her family ate on it every day and it wasn't particularly babied. It looked fine and it still in use after about 20 years of service. It is probably etched somewhat but people overstate the "horrors" of etching as all it means it that some of the gloss is absent in spots. Bistros and other places with marble counters are all probably etched and you didn't notice. And, of course, marble can be polished if you want to restore it at any point - it's far less expensive than refinishing a wood table.

Most of the wood tables in restaurants that don’t use tablecloths, (like in Gastro pubs), have many layers of polyurethane applied. Water and food drippings bead up on the polyurethane, and don’t penetrate. Theses tables have many more layers of poly than you’d normally find on a floor.

You can apply, or have a professional apply, polyurethane to any table...but, you need to do it in a well ventilated environment...preferably, outdoors. You’ll need to do some research.

Personally, I wouldn’t have marble as my kitchen table. Not because of the etching, rather because marble, like all stones...except soapstone...is cold to touch. Nothing beats the warmth of wood.

Our kitchen table is wood. It has a catalytic finish much like high end cabinets. We use large placemats, and make sure to wipe up any drips. We’ve only had it a few months, but so far, so good.