I have a ProNeb Ultra II for my DD who just turned 3. It's very portable and seems to work really well. (It's our only nebulizer.)
I googled it by the product name and found it selling for about $90 (a little less from eBay.) We have the "pediatric model" which costs a few dollars more. I think the only difference is that the pediatric model comes with a face mask for small children. If you've got a mask with your regular neb, though, you probably don't need that.
Edited to add: Ooops! I just followed the link above and I didn't even realize there were some battery operated nebs. If that's what you're looking for then what I have is not what you want. The ProNeb Ultra II needs electricity, but it's quite small and has a carrying case.