Hi Pit,
Unfortunately I don't know where I bought it. Some common shop I guess.
I've searched a bit yesterday.
Mine has 0,5mm thickness, no name on it.
Thinnest I found online was 0,2mm.
(Sorry guys, I only searched for German shops)
0,3mm quite cheap 2,70€ :
Westfalia
Found 0,2mm at Reichelt but just a big pack for 28€ something .
Don't know anything about this shop but it offers 0,23mm 2 Meters for 4,50 €:
?????
If you search for "smd lötdraht" on e**y you will get a variety of different solder even in different colours.
Eeeeb****y
And one find for U.K.
SMD tin solder
Yesterday I flatten it with a battery (AA or something else which is round) until it was around 0,2mm thick and 0,7mm width.
It's very easy to bend, therefore a bit fragile too.
You can polish it to high silver shine or sand it to alloy like look.
But one thing is uncertain for me.
Most of modern solder have this fluid inside. I don't know if that can harm anything. I guess some lacquer on top can't be wrong.
Even to keep silver shine.
It's fun to play with (take care to buy lead free solder).
Hope that helps.
Cheers,
Knacki