Hello and welcome to the forum. We have a thread on feeding senior dogs here
which might be helpful.
The main thing with arthritis is not to let the dog become overweight. Also, gentle but regular exercise for short periods of time is important. I’m a firm believer in hydrotherapy and if you are interested in this, have a look for practioner in your area. It’s not cheap but well worth the money IMHO.
Regarding supplements, some companies add joint support ingredients to their food but when a dog already has arthritis it might be better to give these separately because the therapeutic dose can be accurately administered. The main ones are glucosamine/chondroitin, green lipped mussel and devils claw. Of these, David mentions the latter as being helpful - see here.
Friends have used Nutraquin +
and found it beneficial for their dogs.
If you want advice about choosing a better quality food for your dog, please let us know what type of food you want and if you want to buy online or from a shop. A bit more information about your dog might be helpful too.