Any good IFR Flight Guide (Jeppesen, AERAD En-Route Supplement, RAF Flight Information Handbook) will have conversion tables.
Failing that - learn a few of them, interpolate between those using 1mb = 0.03 ins. Easy ones to remember:
1013.2 = 29.92 (So 1013 = 29.91)
1016 = 30.00
1024 = 30.24