There is one that folds up flat . I think it's from Prince Lionheart. Safety 1st also makes a fold up tub that looks more like a regular tub. You could also get one of those wire and mesh hammocks and use it in the regular tub or possibly a sink if you have a big one.
The Prince Lionhart Flexibath is fantastic. It is long, but folds flat and has non-slip rubber at the folding "seams." We used a bath sling with it before DS was big enough to sit up on his own, and then used it in the regular tub when he was still pretty little for the "big" tub, but had outgrown the bath sling. You can get it on amazon.com and at buy buy baby, and I have seen it on Woot.com a few times as well. GL!