where can I find MyStarcraft?

In all honesty, not worth using in regular season league games. For MySL or playoff games, they're pretty important to give yourself the highest chance of winning. Remember, each player can equip one of each type (including other) and every item has limited use (which includes all games). I wouldn't recommend buying the most expensive one for anyone other than your star player, and monitors are more of a luxury so prioritize keyboard/mouse.

The "Other" category contains mostly situational and luxury items. On the low-end, they can provide a small boost over a short time for a relatively low cost. On the high-end they can provide a sizeable boost, but only for a short time and at a prohibitively high cost.

The hotpack is not very useful but cheap, so you can spam it on your lower-tier players if you want to. On your better players, you definitely want to use the Ring of Power for important matches or, if you have points to bring up your condition, the Devil Pendant is a good choice as well. Star Beads are just not very useful for its cost and that money is certainly better spent elsewhere. Wristband and Glossy Shades are items you'd want to use on lower-tier players for important matches, or your top players if money is tight.

TL;DR: Buy these types of items to put great players over the top temporarily, not to make your players good. That's what the potion is for.

Rainbow Magic Potion is the way to go. Definitely do not get the Vita Potion; all other potions are very situational, and mostly terrible. I wouldn't field players while they're undergoing Rainbow Magic training, as it really wears down on their condition.