Lack of a spare wheel is not a problem. As Colin B stated in post #4 above, all Mk3 came with a tyre repair kit and no spare wheel
However, the sheet metal stamping under the boot floor is shaped to accomodate a full size spare wheel (or a space saver if you prefer). Spare wheels, sometimes including a tyre are advertised regularly on ebay. After I bought my Mk 3 I was able to get an original alloy wheel + tyre to match those on the car at a very reasonable cost. There are also suppliers who can supply generic space saver wheel / tyre / jack kits. (If you buy just a wheel on ebay you will need to source a jack. Ebay often has genuine Honda Jazz jacks)
The only thing to watch for is some late 2018 and most 2019 cars. The sheet metal floor stamping was changed on these models. There is some space under the boot floor but it will not take a full size spare. A space saver may fit, but will be higher than the boot floor cover and needs a bit of bodging to get a tidy fit