I just buy regular seed packets when they're on sale. I like to find them at the dollar store because you can usually get 10 packets for a dollar. I also buy seeds online - anyplace that sells what I'm looking for. The Tortoise vendors usually sell specialty seeds, like Mediterranean seed mix, etc, however, I've found them to be rather more expensive than buying from seed catalogs, etc.
You can go to tortoise supply.com or Carolina pet supply.com. I have bought seeds for my Russian tortoise through both of them and they were great. they also have seed mixes for Sulcata.
I can't fault Yvonne G.
(She always has good advice.)
I bought mine from Tortoise Supply mostly cause I'm lazy.
The small package still has a zillion seeds in it!
The bonus is that some of it is people food..like radish and carrots.
I get mine from the dollar store also. I am trying something a little different this year. I had some left over seeds from planting a food plot and our lease so I just put it out and going to wait and see.
I may be wrong but I think seeds are seeds. It is how you grow them and the soil you use, and fertilizer that you use that makes them organic. Let me know if I am wrong.
I have a friend who community garden just got certified organic it is not as simple as it seems.He explained it to me that to be true organic your seeds have to be organic in that no chemicals were used in processing the seeds. Also, if you get insects or diseases and you use a product that is not organic you can not sell it as organic.
Seeds mixes are good for the variety that they have. They can be specific to the needs/likes of the tortoise species that you are keeping. If you are looking to shop for organic seeds then I suggest you check out this site: http://www.groworganic.com/
Other than that, I agree with Yvonne, check dollar stores and sales in your area. that is where I get many of my assorted greens to add variety to his grass mixes.