You really can find toy making parts just about EVERYWHERE! For example, check out my recent trip to the grocery store:



The first thing you need to do is be aware of the time of year. Summer yields some of the BEST finds as far as plastic ware goes. I suppose it's all those picnics.

But suppose you're not going for an impulse buy. You've got a mission and a checklist and you're strapped for time. Well, no worries. Here is a list of where you can find some popular toy parts at your local stores, about what aisle to hunt for them, and who, in my opinion, has the best deals.
Bracelets Where found: In the party favor aisle of just about any store, usually located near the gift wrap supplies
Stores: Party City (most expensive), Dollar Store/Tree, Michael's, Walmart, Hobby Lobby
My fav: Walmart. I find they have the most consistent variety. Dollar Store/Tree is also great but you're taking a gamble that the stock will be there the next time you return.
(*I've not had a whole lot of luck at Target. Either they are sold out, or just don't carry them in stock)
Foraging CupsWell here, we should break for 2 different categories. There are the plastic shot glass type, like this ones attached to this swing toy here:

These you can find: your local grocery store, usually near the cocktail mixes
Stores: Party City (most expensive), Dollar Store/Tree, Walmart
My fav: Kroger & Publix, two local grocery stores near me.
And then there's the MINI PLASTIC MUG style of foraging cups, like these:

These are much harder to find and really, I've only had luck finding them at one place and that's Party City. The colors seem to vary depending on the time of year. You'll find the yellow ones around Octoberfest, green around St. Pattys, blue and red around the Fourth of July. If you have your heart SET on these, and you can't find them, I suggest going on line.
Plastic Storage Totes:
Where found: Usually near the office/desk organizer areas
Stores: Target, Office Depot/Staples, Hobby Lobby, your local grocery store (sometimes), Goodwill
My fav: Target, and be sure to check the bargain bins up front FIRST. Often you can find these at 2 for $1. If you don't see them that day, check back and check often. Their stock moves FAST!
Pony Beads:Where found: the arts & crafts section of different stores, sometimes in the toy department
Stores: Walmart (cheapest), Hobby Lobby, Michael's, Joann's Etc
My fav: This is going to be a tie between Hobby Lobby & Joann's because while quite often Hobby Lobby has a Buy One/Get One sale that allows me to truly rake in the savings with those big tubs, Joann's on the other hand has some GREAT NOVELTY BEADS, such as the pink collection of heart beads you find here. I've seen other heart beads in other stores, but they don't quite have that lovely, girly pink in various colors like I love here. Are you a sports fan? They also have baseball beads, football beads, basketball. They've got flower beads, too. Hobby Lobby has a wide variety of beads as well, but those are generally made of glass.
C-Clips aka Learning Links:
I've only ever had luck finding these in school supply stores, such as: The School Box
Otherwise, these can be found on link. A popular brand is called Link N Learn Links and go for about $17 for 500. If you find them cheaper elsewhere, please post where!!!
Cat bells:Where found: pet aisle/cat aisle
Stores: PetCo/Smart, Walmart, Grocery Stores
My fav: Walmart. These have a wider openings that are perfect for clipping on C clips. I haven't been able to find this brand anywhere other than Walmart. The ones they sell at petstores are great but they have tiny holes. And please, as always, check to make absolutely certain there is no catnip inside!!!
Craftlace: Where found: arts & crafts aisle, usually with the beads
Stores: Hobby Lobby, Michael's, Joann's, Walmart
My fav: Hobby Lobby and I use Rexlace. It's not only a very popular brand but right there on the label it says "non-toxic". There are other brands, too, but I am wary of using anything that does not specify it as non-toxic. I imagine it would be safe, but if it doesn't say, there's a chance that it isn't--and I am not a chance-taker.