For parts available I hope you mean accessories, attachments and bobbins. These machines tend to last nearly for ever with minor upkeep and a more thorough fix up every 50 years. For replacement parts there are a few sources; ebay being suprisingly good. I think Cidy Peters are still active, for Singers in general Helen Howes in the UK can be worth noticing, but in the case of the 115 you have to rely on US sources. I hope someone else adds to this.
The 115 is a particuarly smooth and light running model, it has a very good reputation. The accessory box for it was probably identical to a 15 at the time, and any low shank feet should in general fit your machine. You will find buttonholers, zigzagger, and more for it. Some modern quilting feet work very well with the old machines. I have had some trouble with jumping feet; even though they technically were low shank I manage to ruin them; it is a bit of trial and error situation , but you will find some that work both modern and vintage.