Here's one, but it's pricey.

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=3373&N=2002+113232

I have 2 that are similar, but without the wheeled part.  One is leather
Samsonite, one is heavy-duty canvas, both look almost identical at first
glance

There's also this one, but it's even more expensive, and made more for
taking Fufu and Fifi on a stroll thru Central Park West ;) :

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=12195&N=2001+113770

Congrats on your new owners! Multiple cats are fun! Though
sometimes they come with issues, but yours sound like a great
match.  I was curious about carriers on wheels and found this link.  
Mega mega expensive though!

http://www.cfa.org/catalog/carrier.html

Most luggage stores sell collapsible luggage carts that you could strap
your carrier onto.  I know the Travel 2000 chain used to sell them,
they're usually in malls.  I think they were somewhere between $50-$80,
but remember you can re-use it when you're travelling with suitcases :)

Something like this, for example:
http://www.irvs.com/results.php?ManuNo=306&engine=adwords&keyword=luggage%20cart

--
Jennifer

I am in a similar situation as you.  I have two cats, and I bought two of
these:

http://snipurl.com/gy0z

For me, it was easier than stacking their former hardside carriers on a
luggage cart, then unstacking the whole thing to put them in the car, re-
stacking them to get them into vet, well, you get the picture.  The
wheeled carriers are sturdy enough for my two large boys, who weigh 15
and 19 lbs, respectively.  The wheels are detachable if desired.  The
downside is that you still have to lift them up and down steps, but I
find that very manageable.