Thing is, I just got a third PC (a laptop) that does all my C64 xferring better than my old secondary PC. So what I want to do is put DOS on that PC, but then there's the bloody soundcard issue :(

Is there a soundcard being made today that is compatible with the old Sound Blaster cards, or will I need to dig up an old one?

Doubtful that they will be 100% compatibile with DOS, really. SB1 is a technology of the past = obsolete in the eyes of companies that manufacture the stuff. Same goes for DOS.
And since laptop SPU's are not replaceable, you are stuck with it.
You might try FreeDos, it might cope with your soundcard a bit better.

Oh, the lappy is running Win98, and it shall stay running Win98 ;)

FreeDos might handle my soundcard better, this is true. Just wonder if the games will ;)

Kinda funny fact tho, I have better luck with that dodgy Win98 version of VDMSound on my lappy than I have with the WinXP version on this machine ;)

I would be surprised to see or hear that a "modern" soundcard can't be compatible with Soundblaster. For me, it's way more a compatibility issue with "modern" OS like XP, thus why we use VDMSound and/or DOSBox.