Monster Book Boss Card Costs

Is there a list of what level the cards become avalible to buy?

You can do that yourself by looking at hidden street keeping in mind that you have to be within 10 levels of the monster.

But here's a list I put together.

Mano - Level 10.
Stumpy - Level 25.
Deo - Level 28.
King Slime - Level 30.
Seruf - Level 35.
Faust - Level 40.
King Clang - Level 45.
Alishar - Level 46.
Timer - Level 49.
Mushmom - Level 50.
Zombie Mushmom - Level 55.
Dyle - Level 55.
Zeno - Level 55.
Lord Pirate - Level 60.
Nine-Tailed Fox - Level 60.
Tae Roon - Level 61.
Papa Pixie - Level 64.
King Sage Cat - Level 67.
Jr. Balrog - Level 70.
Frankenroid - Level 71.
Elliza - Level 73.
Kimera - Level 75.
Snowman - Level 80.
Crimson Balrog - Level 90.
Manon - Level 95.
Griffey - Level 95.
Pianus - Level 100.
Ergoth - Level 105.
Leviathan - Level 110.
Papulatus Clock - Level 115.