I'm certain it can be done, but it may be a bit tricky. If the slides are
the same height as regular 35mm slides, you should be able to scan them
in an ordinary 35mm film scanner. The trick is that at least some
carriers (such as the one for the Minolta DiMAGE Scan 5400) won't take
slides that are wider than a regular 35mm slide, so you'd need to
literally hack the carrier to fit the slides. One scanner that I SUSPECT
(but don't know for certain) would work is the Polaroid SprintScan 35
series. These take slides singly, rather than in a carrier, by inserting
them in the top of the unit. The way it works, you SHOULD be able to scan
each side of the image (if you want both) separately, by inserting the
slide sideways (so that the other side of the slide sticks out the top).
I've not tried this, though. You can get these scanners used for $25-$70
on eBay, so buying one wouldn't be a huge investment. (FWIW, I happen to
have one up on eBay at the moment; mine's auction #3856672313.) I'd also
expect most flatbed scanners with transparency capability to work,
although again, the standard carriers might not work, so you might need
to do it without the carrier.

Another approach would be more destructive: Take the slides apart and
mount the individual frames in standard 35mm slide mounts, or something
similar to them (with smaller image areas, if the individual frames are
smaller). You could then scan them in anything that can scan slides, or
send them to a slide-scanning service. If desired, you could then reverse
the process, although that might require new 3D slide mounts if the old
ones are damaged in the process.

A third approach that comes to mind is to use a slide-duplicating setup
to photograph the slides onto ordinary slide or negative film, and then
scan that. (Or you could photograph them with a digital camera.) This
should be fairly straightforward if you've got an appropriate lens. If
not, the necessary hardware might be a bit pricey, although you might be
able to find something decent on eBay or some other secondhand source for
not too much. Renting a lens might also be a good option.

As to labs that'll do the work, I have no specific pointers. I do know
that my father used to get prints made from his stereo slides, so it seems
likely to me that SOMEBODY can still do it. Try calling some pro labs in
your area.