We all know that one of the best things about flying is all the kit you get to have and all the gadgets available but it probably is best not to be in too much of a hurry.
If you're after saving money (after all, money saved is money you can spend burning holes in the sky) then I'd go for eBay. You should probably buy it bit by bit however if you buy everything your heart desires on Ebay you'll still have paid a third of the price you'd have paid to any of the companies mentioned (probably a quarter of what you'd have spent at one or two of them).
That's my theory anyway, put forward in full knowledge of the fact that if you try to save money in aviation you'll end up spending three times the number you first thought of.
Good luck.