From handy book of conversion tables (there are also a few pretty good conversion tools on the net):
1 mbar = 0.0145038 lbf/sq in
As for a pressure sensor to measure at the pitot tube, are you trying to get airspeed?
On the helo I'm testing on at the moment we have a differential pressure sensor for airspeed recording (connections to both pitot and static) and an absolute sensor for static (altitude) recording.
We used Setra transducers, but the ones we have are only good for up to about 200 kts (differential transducer) and 10,000 ft (absolute transducer).
Our required pressure range (for airspeed) was calculated using:
q = 0.5 * atmospheric density * (velocity squared)
(Difficult to write equations in here isn't it?)
q = Dynamic pressure (lb/ (ft squared))
v = Velocity (ft/sec) (ie the max velocity measurements are to be taken at)
and atmospheric density is in 'slugs/ft cubed'
If you want it in PSI mulitply answer (which is lb/ft sqd) by 0.00694444 to get lb/in sqd.
For atmospheric density we used sea level value (0.002378 slugs/ft cubed). Though I imagine this would still be the worst case (highest atmospheric pressure) for the total pressure at the pitot as well, since you start of with the highest atmospheric pressure, and then add the effects of airspeeds onto that.
As I said this was done for a helo, so I can't say it will any good for faster aircraft, as I'm not sure how compressability would affect the calc (we ignore it in helos, it makes things much easier, and its negligable anyway at our sort of speeds). You don't say what speed range you are looking to measure over, but I hope the above helps.
It might be worth asking on the Flight Testing Forum, I'm sure some the guys there have also had to work this sort of thing out for instrumentation purposes.
Edited to add:
Try this website for converting stuff to other stuff-
http://www.calculator.org/properties.html