Notice that that page says the future unreal conditional looks the same as the present unreal conditional. On, that page there is the note
EXCEPTION If I were ...
In the Present Unreal Conditional, the form "was" is not considered grammatically correct. In written English or in testing situations, you should always use "were." However, in everyday conversation, "was" is often used.
If he were French, he would live in Paris.
If she were rich, she would buy a yacht.
I would play basketball if I were taller.
I would buy that computer if it were cheaper.
I would buy that computer if it was cheaper. Not Correct (But often said in conversation.)
I would not go so far as to call the use of was here incorrect; I would call it informal.
So, to be formal, and on a test, use
If I were not busy tomorrow, I would come with you.
To be informal, you can absolutely say:
If I wasn't busy tomorrow, I'd come with you.
Notice that in informal, everyday conversation, native speakers are also going to use those verbs contractions.
So realize that you will hear and read native speakers using was instead of were, and this is considered informal. The only people who consider it wrong all the time are called pedants (or learners who learn the rule and then say it must be used all the time).
The only time If I was sounds so bad that it is considered substandard is
?? If I was you,....
but even this is used by some native speakers, as part of their normal speech. I would definitely avoid using this on an examination. As a learner, if you say If I was you,... you will not sound natural and people may wince.