That is not a sponsor letter. That's a welcome letter. Don't get confused with sponsor and welcome letter. To sponsor, there is a income requirement. To write welcome letter there is no income requirement. They just need some address and person info where you can go and stay temp upon arrival at US.
There should be lots of consultancies at Nepal who can help you with this. Try to find some near or far relative or friend or anyone who is currently residing at US. Ask for welcome letter and don't scare them by saying sponsor letter.
If you don't have anyone...inbox me. I will see what I can do for you.