I get most of my stuff from pinballlife. The rubber should feel kind of like an eraser, it should be soft and sticky kinda. If its hard and slippery, then it's probably old.

Changing rubbers makes a huge difference. I like white rubber the best usually as it seems to be more bouncy and causes less grime on the playfield. The ball will have much more english and the game will be way more fun with ALL new rubbers all around...

If the rubbers are "a little" stiff then change them. You can also keep an eye on them and when they start to get dirty (use white ones they have better reflex and bounce plus you can tell when your Pin needs a good wipe down...and maybe waxin) you can clean them with novus one and then replace them when they loose their bounce. Another good tell is if you remove them and they do not change form (look like they are still on the posts) change them. You can keep them on the Pin but they are not expensive and new ones will make your game play better. But bottom line is it is really a judgement call and what you prefer.
You can buy them from a number of places, Marco, Pinball life, Pinball Resource, check the sticky thread for a number of companies you can use. Good luck.