Cell Cycle and Cell Division

Mitosis occurs in actively dividing (meristematic) cells.
The root tip has a region of apical meristem where cells are dividing continuously, so many cells in different stages of mitosis can be seen in a squash preparation (e.g. onion root tip).
Anthers (and ovaries) are used to study meiosis; mature leaf tips have few dividing cells.
So the best material for studying mitosis in the laboratory is the root tip.

The mitotic spindle is made of microtubules, which radiate from the centrosomes (or spindle poles in plants) and attach to the kinetochores of chromosomes.
Microtubules are hollow tubes built from the protein tubulin ($\alpha$ and $\beta$ tubulin dimers).
Actin forms microfilaments (involved in cytokinesis in animal cells), actomyosin is the contractile complex of muscle, and myoglobin is an oxygen storing protein of muscle.
So the mitotic spindle is mainly composed of tubulin.