You say you don't have space for anything bulky, so here's what I suggest:
If you have space for the bath ring, then find a Children's Orchard near you and see if they have any in stock. That's where I bought the one for my son, and I only paid about $5. (Incase you're unfamiliar with Children's Orchard, they sell previously owned, gently loved children’s merchandise.) Because these rings are only a transition aide, I would HATE to pay full pop for 2-3 months of use. Once they can stay seated in the light current of moving bath water, its usefulness has expired. That is the point where they start trying to get out of it.
If you really don't have room for it, then go to the kitchen sink. As another mom pointed out, typically your kitchen sink is the perfect size for kids during this transition. (And a REALLY nice break for your back! (O; ) If you go this route, BUY the no slip tub mat, NOW! Having it too soon will cause no negative consequence. It will give you the option of doing a “test run” periodically. My test runs were done in the shower while I bathed. He LOVED holding the shower wand and “getting Mommy” with it. He also loved filling and dumping cups of water.
I learned the hard way, NEVER allow a child to be in the tub with out a slip mat! When my guy was 9 months I had him on the shower floor. I closed my eyes to rinse my hair and he tried to climb up the glass door. Tile floor slippery, glass door slippery, tinny hands slippery= baby slip, eye bleeding, trip to ER and 3 stitches in eyelid. I felt like SUCH a LOOSER of a mom. As they say: it only takes a second to have an accident. Nothing like calling your neighbor to help you with your naked bleeding child while you’re naked too! Oh well, I guess that’s part of why you want to have friendly neighbors.
Anyway, I hope this was helpful. ~J.