Welcome to Librarium Online!
Anyone know of a spray paint that could substitue for GW's spray undercoat?
there are loads of them. Not only are they cheaper, but they come in more colours so you can get a basecoat that in many cases you just have to highlight. If you are in the UK, check out your local hobbycraft for a pretty broad selection.
I use army painters spray and it's great. You get so many colors to choose from and they are also cheaper than GWs.
I second the other guys, Army painter all the way, they've even got a Platemail colour so GK's would be relatively easy too. As to where you can get them depends on your location, my club has a shop too, so we can get them easily from there (Hornchurch) and in London you have Dark Sphere on the south bank.
I buy the cheapest flat primer I can find. Never failed me yet.
Duplicolor sandable primer. This is a little under a third of what you will pay for GW's primer, half of what you'll pay for Privateer's primer. And it is definitely one of the best primers available (in my opinion, better than either of the two I mentioned).
Prices on hobby products are not always based on quality. Often, the same product sold at a game store will cost quite a bit more, simply because you have such smaller sales numbers. Just look at those flat beads people use as counters- get those at a larger shop and you'll have huge bag for $1.
Duplicolor is an automotive brand, so you can find it at automotive shops, or in automotive sections of department stores.