Here's your options:
1. For this one hunt grab a box or two of bismuth #4s from Cabela's and go have fun. That would be the cheapest/fastest solution for one hunt.
2. Take your A5 to a gunsmith, have it reamed to modified and shoot nothing bigger than #2 steel. Cheapest solution, but it will take longer than a week. I'm assuming that's it's not a collector if your taking it duck hunting.
3. Watch Gunbroker until one pops up. More expensive and it will take longer than a week to get it.
4. Hit the barrel exchange websites. You'll get it in a week, but you will pay through the nose.
5. Call Jack Ammons in Oklahoma. He may still have some Hastings aftermarket A5 barrels. Last I heard he had 34" and 26". It will be expensive, but the Hastings barrels are very stiff and pattern awesome (even pattern, consistent shot to shot and tighter than a stock browning barrel) with high speed steel.
Be prepared to spend $350 or more for everything but option #1 & #2. Unless luck is one your side. Here's an example of an A5 standard for sale now:
http://www.gunbroker.com/Auction/ViewItem.asp?Item=120375183Remember when you go to buy your new barrel that A5 standard and A5 light barrels will not interchange with an A5 Mag.
In my experience, Invector+ barrels are hard to find. Standard Invector barrels are cheaper and easier to find.