Hey!
hard hat bonnets are very different than soft bonnet dryers - not sure which one you wanted...but if you're interested in soft bonnets there are a lot of old threads about them. I seem to remember people mentioning :
1. spaceman dryers (looks like this
http://www.amazon.com/Belson-Ionic-Bonnet-Dryer-Gh2138/dp/B0012DOK9Q) or the ion ionic silver one that's basically the same thing.
2. the old vintage GE hair dryers (look like this
http://cgi.ebay.com/Vintage-GE-Portable-Deluxe-Bonnet-Hair-Dryer-with-case_W0QQitemZ250388962238QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item250388962238&_trksid=p3286.c0.m14&_trkparms=72%3A570|66%3A2|65%3A12|39%3A1|240%3A1318|301%3A0|293%3A1|294%3A50) that people get off ebay. If you go this route, watch the prices for about a week so that you get a good dryer at a good price
3. the classic gold n hot (like this
http://www.sallybeauty.com/Soft-Bonnet-Dryer/SBS-255282,default,pd.html)
there are others...
I think the main issue for most people was how well they dry the hair and how big the bonnet is to fit big rollers. I have both the vintage GE and the silver spaceman. Both are big enough for the purple rollers, the spaceman bonnet is larger though. I use the spaceman when traveling because it is so small. Both work very well.
HTH