Hi Jane.
You will probably have to order your Kuna in advance too. Speaking from experience you would have to give them a couple of days to order in, although you may find some of the larger banks keep a supply in.
I didn't do the Mostar trip last year when I went, but you can't order the currency there out of the country. I believe, from what people said that they will accept the Euro and kuna there too, and there are ATM's anyway if you want to exchange a small amount. Lunch is included in the trip, so you may only need a small amount for shopping.
Yes, to Euro in Montenegro. Again you will only need it for lunch and shopping, and unless things have altered you only get a short stop (about a hour and a half) off the coach there anyway.
Have a great time, and give my love to Cavtat. 🙂