The best place to get bulk grain is at Mother Earth.  Because it is human food grade, it's likely quite expensive...I really haven't checked prices.  They get stuff in 50# sacks to put in the bulk bins.  I know they will sell the bulk bags because I bought a huge bag of wheatberry from them a few years back.  
We could check with someone like the feed store in Alachua that makes their own feed.  I don't know if Seminole Feed makes their own anymore...they used to.  
Here are a few others:
Swan's Feed Mill
8916 Fort King Rd
Zephyrhills, FL,
33541-7421
813-782-6969
Southeastern Minerals, Inc.  229-243-5202
J.B. Davis & Company          770-497-8000
Manna Pro Corporation          813-620-9007
Bernie Haskins Company          386-462-1389
Franklin Minerals                  352-629-7997
Purina Mills, LLC                  863-425-5544
Howland Feed Mill, Inc.          386-364-1370
Elanco Animal Health          229-241-7522
Columbia Grain                          386-755-7700
Southeast Milk, Inc.                   800-989-4717
Ocala Breeders Feed & Supply  352-629-8686
Cargill Nutrena Feed Division  407-846-4169
Walpole, Inc.                          863-763-5593
Westway Feed Products          800-940-7253
Seminole Feed                          352-629-2150
Tropicana Products, Inc.          941-742-2531
Gator Feed                          863-763-3337
Dixie Feed Mill
1280 Highway 97
Molino, FL 32577
850-587-3179
Name: Howland Feed Mill
Street: 9743 US Highway 90
Live Oak, fl 32060-
Phone: (386) 364-1370
Don't know if any of these places sells bulk grain...but it's a starting place.  Please post whatever you find out!!