"I'll go buy some tomatoes" sounds natural to my (American) ear as a statement of what I plan to do in the future. Unless otherwise noted, it usually refers to the near future.
"I go buy some tomatoes" sounds natural to my (American) ear as a hypothetical. For example:
"Suppose I go buy some tomatoes, and they all have worms. What should I do?"
Suppose my friends and I are playing Dungeons and Dragons. (Dungeons and Dragons is a game where people pretend to be a team of adventurers.) The Dungeon Master asks me what I want to do next. I tell him, "I go buy some tomatoes."
In all of these examples, the word "go" is optional. The word "go" helps imply that the proposed action is in the near future.