What is a substitute inner tube for 27x2.125" tyres (in Prague)?

Tl;dr: use 700c or 29"

27x2.125" is very rare. 27x2⅛" would be more likely, and wouldn't quite be the same thing, but for tubes the answer would be the same. Generally though, 27" tyres are narrower than this, up to about 40mm or 1 1/2"

Schwalbe says that for 27" tubes you should use 700c, which are very common. The bead seat diameter of 700c is 622mm; for most 27" sizes it's 630mm. As your tyres are on the wide side, you'd want to use 29x2.1" (as 29" is a marketing name for wide 700c)

Tubes are really rather forgiving. Examples: I'm currently running a 26" tube in one wheel of my 29er hardtail (a pain to fit, but I'd taken the wrong tube with me and had to get home); I've replaced a tube with a puncture only to find that I'd been riding one old bike for a year on a 26" tyre with a 24" tube.

Note that the width is also given as a range, so one tube might be suitable for 1.5-2.4" widths. Schwalbe's SV19 is a common example tube, and is specified as suitable for ETRTO diameters from 584mm (27.5") to 635mm (28x1 1/2"/700B) and widths from 40-62mm. I wouldn't choose it for the very largest diameter and width specified, but it would work.

For info, tyre sizing systems are bizarre - 1/4 is not the same as 0.25 for sizes in inches, and 29"=28", while 27">27.5" (and can be bigger than 29").