Sometimes TomTom do sell maps for some of the older models on SD card, but they have always been at least one map version out of date.
The standard way to buy the TomTom maps now is by download to your own SD card or the internal memory of the TomTom (if fitted).
Unfortunately, according to the model chart our moderator 'mvl' compiled, the Rider 1 and Rider 2 have no internal memory and according to Tomtom's own chart
here it says those two models can only accept standard SD cards and
not the SDHC cards that you see more often these days.
Unless you can find one of the very rare 4GB SD cards, that means you are restricted to a 2GB "plain vanilla SD" card and will therefore be limited in which maps you can buy. The "Home" program or the TomTom map shop page on their website should advise.
If the spreadsheet from mvl is correct, and the unit has no internal memory, that means the TomTom's operating system program also needs to be installed onto the card. You will need to get TomTom Home to do that for you by selecting "Update my device".