Yes, you can buy the trekking gear required for the Everest Base Camp Luxury Trek after landing in Nepal.
You can buy almost all the gear that you may need in Kathmandu and Pokhara. If you lose a thing or two during your trek, you can also get shopping opportunities in Namche Bazaar, the highest trade center in Nepal.
Note that the things you buy in Nepal might not be of the same quality as the ones you buy from your country. But that doesn’t mean these trekking equipment aren’t usable. You can get a comfortable and enjoyable trekking experience with the gears available here.