On the assumption of 'cheap-and-chearful', I was about to suggest used Bionicle, which I see in charity/thrift stores regularly, but those will be rather large.
Another is some of the smaller Mechwarrior, which would still tower, but not so much...
If you aren't picky, they can be not-too-dear, especially if you find a nice pile for sale. You can always dump the vehicles and troops. *hint hint*
Doug