Resources in space, and their distribution I'm making a video game in which the economics of an interstellar civilization is important.
What I'm wondering is, how are resources in space distributed?
Since everything ultimately comes from stars, and everything past iron comes from supernovas, I assume an area with more stars blowing up would have more precious metals eventually condensing into planets. But do certain areas of the galaxy have more nova-capable stars? Would core-ward systems have more metals on them, since there's more stars closer together? Or would every planet in the galaxy have more or less the same loadout as any other? Would it only be possible to mine metals on planets that have water, since that's what makes metals condense into veins?
Furthermore, are there any natural resources that might be found on alien worlds, or in space itself, that cannot be found on Earth? Would it ever be feasible or desirable to "mine" a nebula of its gasses? I've heard supposition of there being pockets of antimatter floating about; though that raises the question of how you're ever going to store the stuff.
 A: There is in general a metallicity gradient, just as you suspected. Take a look at this paper, especially the figures. They plot metallicity as a function of radius, and it does increase toward the core (we are $8~\mathrm{kpc}$ from the core). In case you are not familiar with the notation, $[\mathrm{Fe}/\mathrm{H}]$ is how astronomers measure metal content. It is defined as the (base-$10$) logarithm of the ratio of iron to hydrogen concentration, with the $0$-point set at our Sun's abundances. So a value of $1$ corresponds to $10$ times the iron per unit hydrogen that our Sun has.
Iron is used as a proxy for all metals1 because it is easy to observe. The paper actually shows some other metals independently. Honestly, no one could fault you if you used poetic license to have metal abundances not be in the same ratios everywhere, since we don't really have a great grasp on them anyway.
By the way, spiral galaxies are more complex than a simple 1D plot reveals. In addition to more star formation/death at the center, there is more in the plane of the disk than off the plane, and more in the arms than between the arms. There can be infalling dwarf galaxies that bring a small, concentrated burst of metals to a region. There can also be streams of infalling "pristine" gas that has never before been in a galaxy and therefore has essentially no metals at all.
As for what is found in planets, that depends on how the planets formed. Obviously the planets in our Solar System have different compositions despite coming from the same cloud of gas. Probably the biggest effect is that of the ice line. Planets that form far from the nascent star will have access to lots of water/methane/carbon dioxide/etc. ice, allowing them to form with more ease (that is, they tend to be large and gaseous) and to be somewhat diluted when it comes to heavy metals. Inside the ice line, you expect small, rocky worlds to form, rich with metals (all the hydrogen and helium escape the low gravity).
As for whether you need water to have veins, I don't know for sure. Perhaps this question can help with that.
Mining a nebula would be very tedious. The densest of nebulae have something like $10^6$ particles per cubic centimeter, the vast majority of which are hydrogen or helium in some form or another. Compare this to the $3\times10^{22}$ or so water molecules per cubic centimeter in the oceans, which you'll note we are not filtering for precious metals due to how inefficient that would be. Or better yet, compare it to the $3\times10^6$ particles per cubic centimeter found in the "ultrahigh" vacuum at the LHC. The only reason nebulae look like they have substance is that they are so big you see a large column density.
As for antimatter, unfortunately there really aren't pockets of it. If there were, we would see gamma rays emitted from the interfaces of these regions with matter regions. This is the type of thing you're better off generating in a lab.
Hopefully you don't get too bogged down in these details. Giving up some physical accuracy for more fun is definitely a worthwhile trade.

1 "Metals" $\to$ "anything with $Z > 2$" for astronomers.
