Find a local fabric store. They often sell patterned fabric with things like a star background (some are cartoonish - but some are realistically patterned).
If a local source doesn't have it - you can get a couple of yards from a source like this one:
http://www.spoonflower.com/fabric/339848
The wife messes around with sewing a bit - so we end up ordering from them every couple months for different things. They provide pretty good service.
After that you have wargame related companies like Corsec who provide similar materials:
http://corseceng.com/fabric-mats/
Finally you can go to one of the various space agency telescope sites, like the Hubble Site...find a high resolution image you like....take that to a print shop that does wide format printing...have them print you off a few feet worth:
http://hubblesite.org/
Of course, you can also paint your own or poke around to see if you can find a poster large enough (they tend to be smaller than 3 feet though...). The Hubble Site has a print shop - though I haven't looked into details of what they offer.
Once you have your base done, you can add some terrain too if you like. Things like asteroids and what not that add some variety to an otherwise open playing field. I have had good luck messing around with spray foam to get some useful irregular shapes (as well as plenty of blobs that just get tossed...though once you get a hang of it - it is handy). Also hacking away a bit on thick chunks of extruded insulation and then using a hot poker to melt craters into them.
The spray foam can be painted with anything straight away (it is a urethane foam - so aerosols don't impact it). Insulation foam of course carries with it the normal issues when painting (aerosols can melt it...though sometimes in helpful ways).
Plop those on flight stands and now you have an asteroid field for you Imperial and Rebel fighters to zoom through.