On why nail polish is like twinkies...

Nail polish is a little glass bottle full of chemical goodness. I know a lot of us, myself (in the past) included, look at a thick gloopy dried up old bottle and think - this polish has gone bad. Then thrown it away.
Don't think that way any more. Nail polish is almost NEVER unsalvageable.
If it's too thin...leave it sitting open. Some polish will evaporate, but the liquid should thicken.
If it's too thick - even if it looks dried up and solid...add thinner and balls.
I have two types of thinner.
Seche Restore is a thinner that has toluene as an ingredient. It is made for Seche Vite topcoat. I like using it in old polishes that are not big 3 free (containing toluene, dibutyl phthalate, and/or formaldehyde) since there's no point in adding big three chemicals to B3F polish. I think the b3f and the non-b3f are best kept to themselves so you don't have chemical compatibility issues.
I personally have three different B3F thinners -
-SpaRitual makes one with the ingredients: Butyl Acetate, Ethyl Acetate (it's called vegan, I guess because of the omission of petroleum-based Heptene?)
-Nubar makes one with the ingredients: Butyl Acetate, Ethyl Acetate, Heptene
-Zoya makes one called Renew with the ingredients: Butyl Acetate, Heptene, Ethyl Acetate
They all contain similar chemicals, but with differing formulas. It's really about what works best for you and what sort of bottle design you prefer.
You can get any of these from their respective company sites. If you have a good beauty supply nearby they may carry one or more brands of thinner.
Sally Beauty Supply carries a thinner called Beauty Secrets with the same ingredient list as Nubar's (Butyl Acetate, Ethyl Acetate, Heptene).
As for balls, many ladies I know swear by zinc coated bb pellets made by Daisy, which can be purchased at Walmart or on Amazon. Myself, I have the Daisy bb pellets and I've had problems with rust and discoloration in the nail polish bottle.
I have switched to stainless steel ball bearings from eBay. I bought 316 1/8" diameter balls from a seller called tool supply and they're a little smaller than bb pellets, but I think they'll make a fine replacement.
I usually drop in two balls and add thinner 5 drops at a time. Roll or shake the bottle after each infusion of thinner until your polish is restored. You can swatch it on a piece of paper to test it's smoothness. Don't get too aggressive with the thinner, an overly thinned polish isn't a great improvement over a too thick one.

NOTE: Please NEVER use acetone as a thinner. It ruins the polish.