As for explosives, you'll be surprised how many bangy things you can make at home or get your hands on relatively easely.
First of all, any firework's dangerous, especialy when it's packed togheter in a pipe or something. It's not too unstable though, so short-term storage and handling shouldn't be a problem. You might want to cover the bmb in vaseline to make it water-tight.
Another easely obtained explosive that can be used to make a bomb is Amonium Nitrate, which is common fertiliser. I don't know if it's good for a pipe bomb, because it's usualy used to load a van up, which is capable of blasting away pretty much an entire street full of zombies. It has the added advantage of requiring an initiating explosive, which makes storage and transport very safe compared to toher bombs.
Potassium Chlorate, which is also usefull for pipe bombs can be made from bleach and potassium chloride (sodium-free salt). Not too safe for storing though.
Then there's things like a Crofty bomb: Sodium Hydroxide (you can get it as a sink cleaner, Crofty is a dutch brand) mixed with aluminum tape will generate large amounts of hydrogen gas which can cause a pressure explosion folowed by deflagration when it's in a closed container. Only works if made on the spot, but pretty damn effective.
Solidox is sometimes sold in hardware stores and it's a highly oxidising agent. When mixed with something like sugar it becomes an excelent explosive, though not safe for storage. Making such a bomb is best done at the spot.
and then there's nitrocellulose, or gun cotton. This can be made from sulphuric acid, nitric acid (both can be bought as cleaning agents) and cotton. This burns very violently and rapidly and comes in different variations, one of which is sometimes used as rocket fuell. When made well and confined in a container it's a far more powerfull explosive then gunpowder or even TNT. The less powerfull variants are also pretty safe to handle, unlike most explosives (just don't let it come into contact with something hot).
Any powerfull oxidising agent mixed with sugar or powdered aluminium and confined can give a pretty nice bang. Potasium Permanganate fits the bill nicely and is often freely availible as amongst others a pool cleaning agent. Mixing it with aluminum powder from a paint shop will yield a powerfull yet unstable explosive. The same explosive made with sugar instead of aluminum powder is less powerfull but safer for handling. Still it's best made on the spot since potasuim permanganate is safe for handling and so are sugar and aluminum powder unless you'd manage to get a powder explosion out of it...
Much more examples of home-made explosives can be found around the interwebs, most of which contains easy to obtain substances like those mentioned.
I sugest moddeling in at least some of these explosives, if only to give a nice touch of detail to your game.