Any plumbing supply house should have a 1 1/4" wye tailpiece. They are used all the time arount here for A/C condensate drain connections.
That said, I don't like the sound of your dishwasher hookup. There may be problems...no air gap, flex pipe through the wall, etc. These are code issues. If it is working OK for you, we don't argue with success, but these issues may come up later on...like if you were selling the house, or doing other work and an inspector was called in.