You realize it's illegal to decode common carrier paging systems or messages not intended for you. I don't see what thrill people get out of decoding this information. Use that SDR dongle for something that is worthwhile.
First of all, POCSAG is all but obsolete. No common carrier in the US uses it anymore so your point is moot.
Second of all, POCSAG is a legal mode for amateur radio. Kantronics even included it at one time in their software for their TNCs.
As for the OP's question, the secondary market is about the only source left. We were running our own POCSAG system here at the fire department until we moved to other systems. We were buying our pagers direct from mainland Chinese manufacturers up until 5 years ago. We were on the 450 Mhz band. We sold all of our spare and used pagers as surplus and were bought for scrap value. We used POCSAG to control all our Civil Defense sirens as well as paging.
Keep looking, they are out there. As for how to identify them readily, I do not know although Motorola had a few POCSAG only pagers with specific model numbers. Finding software to reprogram the channels they were on can be a pain in the rear. I know there were several ham clubs that used the pagers to notify them of emergency incidents. Some of those pagers may be floating around.
I admire your curiosity and encourage you highly. All the naysayers on here obviously do not know what they are talking about and are only too glad to tell you why it won't work or is illegal.
Craig