Can I purchase data for my iPad to use when away from wi-if? No mobile carrier

Should you have a WiFi+Cellular model iPad, you can either obtain a physical DataSIM from a Cellular Carrier of your choice, or for recent models of iPad with an eSIM you can enable Cellular directly from iPad settings:

Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support

While prepaid plans are commonly available for iPhone, prepaid data plans are perhaps less commonly available from primary carriers. However, there are eSIM plans from MVNO network operators that offer short-term or prepaid data plans for iPad - these being aimed at occasional use and users that require short-term Cellular data when travelling (e.g., GigSky, ReadteaGO and Truphone).

For recent iPad Cellular models with eSIM, these plans can be activated directly from iPad settings; for older models, you'll need to plan ahead and obtain and activate a physical DataSIM.

If your iPad does not have a SIM slot, then you have a WiFi Only model of iPad. WiFi Only models do not have any Cellular capabilities - and hardware cannot be retrofitted.

Whilst a WiFi Only iPad cannot be transformed into a WiFi+Cellular model, you can use the hotspot feature of a mobile phone - or a Cellular Broadband Router (often known as a MiFi Router) - to access Cellular networks.

How to set up a Personal Hotspot on your iPhone or iPad – Apple Support

Did you buy an iPad with cellular capabilities? If so, you would need to contact a cell phone carrier to see what your options are. Each one is likely different.

Supported carriers are listed here: Wireless carrier support and features for iPad in the United States, Canada, and Puerto Rico - Apple Support

~Lyssa