First possibility: Search online at eBay for an old Skyper etc. and adapt it for HAM-frequencies. Be aware, that there are pagers with different frequency ranges (e.g. also for 2m).
Second possibility: Cheap programable pagers are distributed by AlphaPoc, e.g. the AlphaPoc 602R (model X4 437-441 MHz). The AlphaPoc is hand-programable for the most important settings (RIC, rubrics, frequency).
Third possibility: A high-end-pager like the Swissphone s.QUAD X35 or the LRS Birdy WP Text Pager.