I live in Apple Valley and did a lot of calling around. There are no places south of the river that will fill. Here is what I found, and all of these prices are for 5lb.
Jason's Dry Ice in St Paul will fill for only $10, but you have to leave it overnight. If you want to wait, it's $25 to fill. Hours are 7-3 M-F only.
Praxair in St Paul is $20 to fill your tank while you wait. Hours are 7-4:30 M-F
Oxygen Service Company in St Paul wil fill for $18 while you wait. Hours are 6:30-5 M-F.
Minneapolis Oxygen Company off Dowling and 94 will fill for $20 while you wait. Hours are 7:30-5 M-F. Exchange is only $16.
There are likely other places, but I stopped calling once I found one that worked for me. I use Minneapolis Oxygen Company because I drive by every day on my way to/from work. The rest are far to inconvenient for me. When I stopped in at about 3:30pm they had shut down the CO2 filler and would only exchange. I had a shiny new aluminum tank and wanted a fill. I explained that to them and they showed me how they kept shiny 5 lb aluminum tanks behind the desk for just this.
Since they actually fill their own, they get to pick out the nice ones and keep them for people that swap in nice ones. He pulled one out that looked exactly as nice as mine. I did an exchange. As long as it looks as nice, why not save $4 and a bit of waiting.
The way I found them is I just pulled up google maps and searched the area for "gas service" and went from there.