Thanks for sharing your question! One specific option for finding a duffel is REI. You could also buy directly from any of the big-name outdoor gear companies that you’ll find listed in that link. All sporting goods / outdoor gear stores will typically carry duffels. You can also find cheaper, less durable ones at big department stores like Walmart which are enough to survive the trip to Nepal and back. Whatever you choose, make sure that it will be big enough to hold everything that you want to put in it!
I hope this helps, and look forward to meeting you soon here in Kathmandu!