According to the information on Sensirion website, I find that SHT15 & SHT75 nearly have the same specifications, except for size, mount type and price. (SHT15 also needs 8s for RH response).
You're quite right about the response time. Sorry for giving you a bum steer there!
Be careful about the box you choose too. There's a whole section in the data sheet about how metals and sealants effect the response times of the devices:
"1.8 Materials Used for Sealing / Mounting
Many materials absorb humidity and will act as a buffer
increasing response times and hysteresis. Materials in the
vicinity of the sensor must therefore be carefully chosen.
Recommended materials are: Any metals, LCP, POM
(Delrin), PTFE (Teflon), PE, PEEK, PP, PB, PPS, PSU,
For sealing and gluing (use sparingly): High filled epoxy for
electronic packaging (e.g. glob top, underfill), and Silicone.
Out-gassing of these materials may also contaminate the
SHT1x (see Section 1.3). Therefore try to add the sensor
as a last manufacturing step to the assembly, store the
assembly well ventilated after manufacturing or bake at
>50°C for 24h to outgas contaminants before packing."
Check out Futurlec for parts. They don't carry the SHT's, but they have a HH10D there which is a pretty cheap unit fo experimenting with. You'll need a 555 timer with that one too. The data sheet seems reasonably detailed with an example cicuit diagram.http://www.futurlec.com/Temperature_Sensors.shtml
I've bought stuff from them before and they shipped pretty quickly to Australia.
Hope this is of use to you