First, a big thanks go out to Cliff Piequet at Back Street Brew Pubnow known as Hornitas Brewery who turned me on to MiCervesa. Both he and I have been searching for this source of bulk grain and ingredients going back a couple of years.
I placed a 'group buy' order with MiCervesa because they have a $250 usd. minimum. I was very happy with the service and they have a great inventory of grain. They also sell pellet hops by the pound and dry yeast by the case (38 pgks.) that can not be mixed. The pale malt is $43 for 25kilo bags. If you brew all-grain and you can make the minimum, this company is the way to go.
The other wholesale supplier is Complementos Alimenticios for malt extract. There is no minimum cost amount but the smallest quantity for DME (dry malt extract) is 25 kilo bag at $86 or $1.56 per pound. That's a good price if you're brewing extract or partial mash beers. They also have liquid malt extract. Again, the service is good but you must arrange delivery yourself. For this company and for MiCervesa I used a local courier FridayServices. Very friendly, quick service and they comunicate with you during the process which is unique and valuable here. The last wholesaler that I know of, but have no experience with, is Maltayderivados. They have bulk 6-row grain and are out of Mexico City. I think they are connected to the Modelo group. I believe that the minimum order is 50 kilos.
One thing to know about purchasing from these wholesalers is that they require you to deposit the purchase amount into their bank account and then fax or email the receipt as proof before shipping. They don't have any type of on-line payment system. Some patience is required when going to a Mexican bank to accomplish this task. Take a book to read while you wait in line.
The following are the homebrew retail outlets that are available on-line down here and have a reasonable amount of homebrew materials when it comes to ingredients and equipment. I have not tried to order from them yet so I can't comment on their service, but purchases can be made on-line and they will do the shipping with their prefered shipper. I have had some email communications with them and they are responsive and helpful. The downside to these retail outlets compared to the States is that the costs are substantially higher. So, if you can order bulk, do it. It's just another reason to brew more beer.
I hope that this information is helpful and can save you some time and effort. If you have any questions about this, ask it in the comment section below and I will respond likewise. Also, if you have more information on additional resources for brewing ingredients or equipment in Mexico, let us know. Cheers!