>> We will enter Switzerland through St. Moritz ... Can we get similar stores in St. Moritz
The main problem with buying clothing at your destination is that it can take a lot of time away from sightseeing. The chances of walking into a shop and finding everything you need all at once are fairly slim. You will probably end up traipsing from store to store looking for the right style, size or price, and wasting one or more days doing so!
Nevertheless, you could try the Coop Store Bellevue in St Moritz, or Angerer Sport in St Moritz.
I haven't been into the Coop Store Bellevue St Moritz, but I have been past it frequently in the bus, and noticed they had winter clothing and luggage in the window. That was in early October last year. July is the summer season though, so I don't know whether they will still have any winter clothing for sale.
Another store in St Moritz you could try is Angerer Sport. I haven't been in there either, but they are part of the Intersport group, which is one of the big names in Switzerland for winter sportswear and equipment.
The town buses in St Moritz run complicated routes that are hard to describe, but if you get Bus #3 from St Moritz Railway station, it will go to both bus stops noted below, i.e. Via Salet and Klinik Gut. Stops are announced on the bus, and there is also a visual display, so it's easy to know when to get off.
Coop Supermarket St Moritz Bellevue
Via dal Bagn 20, St Moritz
Bus #3 from St Moritz Railway Station to the stop "St Moritz Bad, Via Salet"
08:00 to 19:00 Monday to Thursday
08:00 to 20:00 Friday
08:00 to 18:00 Saturday
Via Maistra 2
Bus #3 from St Moritz Railway Station to the stop "St Moritz, Klinik Gut"
Monday, Tuesday, Thursday, Friday 09:00 to 12:30, 14:00 to 18:30 (i.e. closed between 12:30 and 14:00)
Saturday 09:00 to 17:00
Closed Wednesday, Sunday
I suggest you look up the addresses in Google Maps, find the relevant bus stops (which are marked on Google Maps), and have a look at the stores in Google Street View, so you know what you are looking for when you get there. That will hopefully save you some time.