You can say, for example:
- Will you be free to help me?
- Will you be available to attend our meeting?
Saying "can you be available to help me?" though I think it is not grammatically incorrect, it does not sound natural, so I would suggest going with one of the other options.
As an additional note, using "would", if you intend to ask for a favor, is also very polite and respectful:
"Would you help me with my homework?"
"Would you open the door for me?"
Regards,
Jaime :)