Most stores should do that, I haven't met any salesmen that wouldn't do that, even the most stuck up their ass did that for under 15$ in my country, 90% would do it for free. If they charge more, walk away in unconcealed disgust.
Just ask around, pick your favorite stores, "express" your lack of interest in prebuilt computers tell them you would want something assembled; if they give you a hard time you will go spend your money somewhere else.
Remember, you are the customer, you say how you spend your money, they don't get to impose conditions on you, your 1400$ configuration will get them over 500$ worth of profit, there is no way in rats ass they should refuse to get a single person to spend 30 minutes of his time putting it together for you.
If not, we'll meet on teamspeak, I'll show you how to it myself!