Smyths, unfortunately, doesn't stock micromasters at all (unless you live in Germany). And I've only ever seen wave 1 in the Entertainer but they don't seem to be that good at getting new stock. There's a Forbidden Planet near me that I've seen wave 2 micromasters in but, again, never anything beyond that. Honestly, online retailers are probably your best bet. I got most of mine from Indemand. However, if you're looking for a larger instock selection, the Japanese retailer Hobbysearch has most of them in stock. Granted, you'll get the Japanese releases but postage, honestly, isn't that bad (especially if you go for the cheapest option, it's usually only a couple of quid but can take up to a month to arrive). Hope this helps your search!
To save you from searching;
https://1999.co.jp/eng/m/search?typ1_c= ... mers+SIEGEHobbysearch currently don't have any stock of wave 1 and laserbeak+ravage but have the others. The wave 4 micromasters aren't out yet. But they do have reservations for wave 4 & 5, as well as Battlemaster Rung and Singe.