Just been into Warhammer World and unfortunately the only battleforce they had remaining between 40k and AoS is the Slaves to Darkness (4 boxes on display so perhaps not the bestselling kit). I checked with staff who said the seraphon set was "long gone" there.
On the bright side there are some other out of print bundled sets in stock there. As of ~2 hours ago they had 3x Looncurse, 1x Realm of Chaos: Wrath and Rapture and 1x Blightwar on the shelves. Some collector edition battle tomes were also there. Khorne, Slaanesh, Gits and FEC were definitely on the shelves. I didn't spot Fyreslayers, Skaven or Beastmen out of the recent releases.
Hope the trip goes well and good luck on the search.
Off topic but something I noted in WHW: It's interesting that the high elf; non-DoK dark elf and dispossessed kits have been moved further away from the bulk of the AoS shelves (on the reverse side of the scenery stand) but the wanderer kits (sisters of thorn/wild riders and eternal guard/wildwood rangers, etc) are still amongst the properly establish factions with tomes. That goes for the heroes that still have WH fantasy packaging and made of metal too. The dark elf kits probably correspond to those with profiles that got squatted in the GHB19 errata though I didn't look for them in the display cabinets to check against.