The particles help grip the lane more to create more friction. Particle balls have a shorter life than reactives and are difficult to resurface or adjust the surface on. Reactive covers have a higher coefficient of friction than earlier coverstocks. Reactives will not damage the lanes as much as Particle balls but will absorb oil faster than particle balls. New urethane coverstocks are seen less and less often now.
Urethane provides a harder shell and less friction than reactives.
No, we cannot assist you in finding a particular ball that you would like to purchase or have lost. Your best chance is to call several pro shops within your state. Usually there is no such thing as practicing too much. However, if you are not using good techniques and you are not learning how to adjust to the conditions then the extra practice could be wasted. If your average is unchanged after all the extra practice or if its declining it can point to the need for outside help.