Either "for" or "until" would be idiomatic in both your examples. "Until" is slightly more pedantically correct, but a native speaker would understand either, and might use either (or the abbreviated "'til", dropping the "un-"). For most purposes, they mean the same thing in this context. The reason "until" is slightly more correct is that "tomorrow" is (in this context) acting as a time, rather than an object (for the arrival of which you might wait) or an event (for the occurrence of which you might wait).
In some slightly modified examples, "for" and "until" would still mean the same thing, but would need you to phrase the sentence differently:
- "It is cloudy. You wanted to go to the beach, but you had better wait until it clears up", but "[...] but you had better wait for it to clear up"
- "I think the supermarket is already closed. You had better wait until they reopen tomorrow", but "[...] wait for them to reopen tomorrow"
However, as a side note, your examples probably do not mean what you think they mean. "You can better wait until tomorrow" means that you are better able (than something or someone else implied by context) to wait until tomorrow. Normally we would say "You had better wait until tomorrow".