...if you don't feel like shelling out exorbitant prices at the makeshift pawnshops or can't find a survivor with a spare dose...
In the game's defense (and I love this game, but agree with most of your criticisms), if you go online and perform well in one episode(?) of Terror is Reality, you can easily net yourself ~$65k, enough to purchase two doses of Zombrex and start you on your way to a third.
I don't normally condone forcing SP gamers to use the MP modes in games, but I felt like the four-for-all matches counted on new game content, moreso than the feel of competition against others.
Edit: Played some more last night. Turns out the price of Zombrex goes up after each purchase. TiR is still a good way to make in-game cash, though.