inverter is DC to AC, not the other way around. you need a 12v (or 13.8v) AC-DC power supply. for instance, if you search radioshack's site for 'power supply' the first three results are 13.8v power supplies, designed to run car accessories indoors, the first one being 15 amp output which should be more than enough for your system.
you can probably do much better on price on ebay or elsewhere, rat shack is usually a ripoff.
I won't flame you for it since you have over 900 posts, but you really ought to read the FAQ where this is covered see post #3
FAQ: Power Supplies Explained (Part 1)
as a cheaper alternative, if your carPC doesn't use a ton of power, you might be able to hack a regular ATX supply and get +12v from it to power your PC; but depending on the ATX supply, it might only be able to provide 6-10 amps; for your epia system that should still be enough, but you should do some quick back-of-the-envelope calculations to double check.