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Welcome to MySwissAlps, Sladman!
You can print the Swiss Half Fare Card at home through http://www.swissrailways.com/ en. After filling out your address, they will show the delivery options. Pick “Online Ticket” from the bottom of the list. They will e-mail the card within 1-3 business days (no delivery fees).
Regular tickets from Milan to Domodossola (the border) can be purchased via the websites listed here.
Discounted tickets from Domodossola to Mürren can be purchased via the timetable (SBB site), but only 30 days or less before you travel. You will need to select the “Half-Fare travelcard” (which is a similar card for Swiss residents). I don’t know another way to buy this specific ticket in advance, but this will work. You can even buy the entire ticket Milan-Mürren this way (no discount on the Italian leg though), but note that the card for residents and the Swiss Half Fare Card for tourists are not identical, mainly when international trips are involved. Hence the option to split the ticket. In any case you can stay seated in Domodossola as you’ll have tickets for the entire trip.
By the way, what do you mean by “Collar” in the title? Did you mean Colmar by any chance? If so, you can buy tickets from Basel (border) to Colmar here.