I have a Combi Twin Sport, a Babytrend Sit n Stand, and a Kolcraft Contours Options Tandem Stroller. I love all three for very different reasons. I had a regular Graco Tandem that I did not like, and have gotten rid of it.
The Combi Twin Sport is a relatively small side-bye-side. It is very easy to push, and I find it much less bulky than either of the other tandems. You can fit a car-seat in it. It's very light-weight, but if one child weighs considerably more than the other (as in the case of a 4 year-old with an infant) it can be kind of hard to push straight, and there is almost no storage space.
http://www.combiusa.com/Products/Item.aspx?Item=46
The Babytrend Sit 'n Stand is very awesome for two kids of very different sizes. It has one complete seat (where you can also put a car-seat) and then a padded bench in the back for an older child to sit. It also has a little platform for the older child to stand on. This is an excellent stroller if your older child is going to be spending a lot of time getting up and down for various reasons, but not for an older child is who just along for the ride, as the bench is not particularly comfortable. Small storage space, but an adult and children's cup holder.
http://www.babytrend.com/sit_n_stand/
The Kolcraft is by far the niftiest stroller I own. The seats lift off the base, so they can both face forward, face each other, face away from each other, etc. There is also a very specific infant-car-seat adapter that comes with it that fits most forms of car-seats. VERY large storage, you can fit several backpacks! Downsides are that it is quite heavy, and tall children won't fit under the short sunshades. Also, there is an adult cup holder, but no children's cup holders.
http://www.kolcraft.com/products/strollers/tandem/149-con...
Good luck with your search!
ETA: Check out Craigs List. That's where I got my Kolcraft and Babytrend.