It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) permitted the one in ihram to wear trousers if he cannot find a lower wrap (izar), and to wear leather socks if he cannot find sandals.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11303