You can pretty much use any keyboard you want with USB. I used to use my tablet with one of those usb thinkpad keyboards - thin and easy to carry around.

For my phone - I looked at some folding Bluetooth keyboards on aliexpress which /look/ nice but at ~$30 it's a little steep for something that might be crap and end up never used/trashed. Maybe on Nov 11th i'll bite the bullet if there is a good sale.

Just search "folding keyboard" on ali.