Refilling a down?

Hello

I have a good down jacket since 7 years ago. The last time I got it cleaned in a specialized shop I found that it lost half of the down filling, but unfortunately I didn't check it at that moment.
It's specially in bad condition at the shoulder and higher back areas because I used it with a backpack.
I would like to repair it myself by adding new down.
Is it advisable to perform small cuts in the inner side (of each affected section), fill it, and sew it?
Or this (sewing with a home machine) would tear the fabric?, Is it better to try to unsew existing sewings?
Where can I buy 850+ down filling?
I've found some at Amazon but they sell 3 pounds of bad quality down for $90+shipping but I only need less than 1 pound, maybe 1/2 pound is enough. I couldn't find it at eBay.

regards

What was the jacket originally filled with?

Do you mean you had the jacket dry cleaned?

I wash my down items myself, either by hand or at a laundromat in a front load washing machine, because dry cleaning ruins the down and it's less expensive than sending it away to be cleaned.

There are online stores that sell 800 fp down, but it is expensive. You'll likely pay 8-12 per ounce.

I would contact the manufacturer if they are still in business and ask them about replacing the down. I know that Marmot will wash and replace down in their bags at a very reasonable cost or over stuff a bag to increase its temp rating and I would think that some of the others may as well.

As far as I know, dry cleaning will ruin a down bag or jacket because the dry cleaning fluid strips the natural oils off the down. I take my bags to an outdoors shop that sends them out for washing.

Source for down. Thru-hiker