Ribbon Rose set, which is almost too cute to use as just a nightgown! I don't think any of the other brands currently have any for sale, but they might not be too hard to hunt down on the second hand market. Swimmer, though not a Lolita brand, has a few really cute cami and bloomer sets.
Victorian Styled Nightgowns
I think that a soft cotton Victorian styled nightgown is just about the perfect thing for a Lolita to fall asleep in! And there are many places online that specialize in reproduction Victorian clothes. VictorianNightgowns.com is exactly what it sounds like, they're the perfect place to find pretty nightgowns. The Victorian Trading Co. also has a few scattered throughout their catalog.
If you don't mind vintage wear, try hunting down something cute and ruffly at a vintage clothing store. Maybe a used night gown might be a little ick for your taste, but you can easily find a ruffly pastel dressing gown to wear over a new nightgown. Of course, vintage wear sometimes takes a bit of effort to make it wearable, especially if it has been hanging around an attic these last few decades. Here is a great 2 part guide (part 1, part 2) on cleaning vintage clothes.
Sewing Your Own Lolita Nightgowns
If you are skilled at sewing, or even hardly skilled at all, you could attempt to make your own Lolita nightgown out of soft comfy fabric. Here is a pattern posted to EGL some time ago, it's a very simple looking design, but very similar to the style of nightgowns Baby often sells! You can also find sleepwear patterns in the various Japanese language Lolita pattern books and bibles. I know that Gothic & Lolita Bible #21 features a few patterns. Or try checking out your local craft store for patterns, many pattern companies like Butterick and Simplicity sell patterns for Victorian and vintage styled nightgowns.