Here are two links. The first is a couple of sentences by Rick Bayless about why he loves his molcajete, and the second is the one he recommends. It holds three cups.

http://www.epicurious.com/articlesguides/blogs/editor/2007/08/i-love-my-mexic.html

http://store.gourmetsleuth.com/molcajete-smooth-basalt-pig-P111.aspx

I really love the Japanese mortar & pestle because the interior texture helps grind things quickly:
http://www.amazon.com/Helens-Asian-Kitchen-Ceramic-Suribachi/dp/B000UEYXL8

I have a few M&P's, but my favorite one is my cast iron mortar and pestle by Typhoon. I would recommend it because it is very heavy, which makes quick work of spices and the like, and will not chip as easily as porcelain or rock. I also was always a little worried about eating the dust that occurs when you grind in a porcelain mortar, I know it is probably inert but it still worried me. This one by Typhoon is my pick.

http://www.amazon.com/Typhoon-Cast-Iron-Pestle-Mortar/dp/B0009XHDP8/ref=sr_1_1?ie=UTF8&s=home-garden&qid=1288544611&sr=1-1

I love my Mexican molcajete. It's made from a black stone, I think it's volcanic, and it's very large, holding about a cup and a half. Because the stone is rough and porous it grinds things up easily and I love to make salsas and serve them directly from the molcajete, which looks really cool on the table.

Here's a jpg.

Hi melissav - I love my tine porcelain mortar and pestle, and I agree that it does not hold much. I did find this porcelain model that comes in a 20 oz. size:

http://www.cooking.com/products/shprodde.asp?SKU=581877&CCAID=FROOGLE581877