The good news is, there are plenty of good solder brands out there. If the Multicore stuff isn't readily available to you for whatever reason, you can also consider products from Kester, MG Chemical, Stannol, Cardas, Chip Quik, etc. And while I've never used it myself, I've actually heard good things about one of those cheapo Ali-Express brands - "Mechanic"[1].
And since this whole sub-thread started with a mention of "silver bearing" solder IIRC, I'll point out that there are a couple of decent options there. I actually like the Radio Shack brand silver-bearing solder, but I don't know if it's still available or not. But Chip Quik have a product in that space[2] and they are a pretty reputable brand so I would expect it to be fine. That said, I don't think the whole "silver bearing" thing is terribly important and I rarely use the stuff myself these days. YMMV.
And since this whole sub-thread started with a mention of "silver bearing" solder IIRC, I'll point out that there are a couple of decent options there. I actually like the Radio Shack brand silver-bearing solder, but I don't know if it's still available or not. But Chip Quik have a product in that space[2] and they are a pretty reputable brand so I would expect it to be fine. That said, I don't think the whole "silver bearing" thing is terribly important and I rarely use the stuff myself these days. YMMV.
[1]: https://www.aliexpress.com/i/2251832830340296.html?gatewayAd...
[2]: https://www.amazon.com/Solder-Wire-Lead-Silver-no-clean/dp/B...