If the character wants to start with piles of explosives right out of character creation I think they would need to pay the nuyen cost for them. Starting nuyen isn't a bankroll they get to go out and spend, it represents the value of gear, implants, etc that the character would have aquired before play started, be it purchased, manufactured, stolen, inherited, found next to I-5, whatever. Its a balance thing as much as anything else.
A character with a good chemistry skill and the right shop could certainly start cooking once play started. They would both need to pay for the shop/facility equipment, and figure a space for it in their lifestyle cost. Some of this stuff may be quite smelly or noisy, so I doubt you would want to do this in your appartment.
Also, contacts might be needed to purchase the raw ingredients without attracting attention. Someone purchasing large amounts of amonium nitrate and fuel oil and shipping it to an address that is not a farm is very likely going to get a visit from Knight Errant. There is a real world perposal that trace materials called taggants be added to things like fertilzers in case anyone makes a bomb out of it to make it easier to trace the source. In the sixth worth with both more advanced technology and magical means available bomb makers would have to be very, very clever or incredibly paranoid about their source materails to avoid having their work traced back to them.