Q: How can I find my id?  I lost my ipad!

Assuming that you have more than one Apple ID (i.e., that it isn't the same one you used to log in to this forum), the following may help: https://iforgot.apple.com/appleid

Not sure I'm understanding completely but if you have several Apple IDs and you know the ID (email) and password for each, if you entered it to your support profile you could log in with each ID and see which one has it: https://supportprofile.apple.com/

The other option (if the device is online) you could try logging in to iCloud.com from a computer with each ID and see if the iPad shows up in the Find My iPhone screen at https://www.icloud.com

I can't think of any other way that you could determine which of several IDs a device was assigned without having the device in hand.

The following has more information on a lost or stolen device: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

That depends on what you did before the iPad was lost.

If you had Settings > iCloud > Photos > iCloud Photo Library turned on, you can log in to https://www.icloud.com/ and the photos would be visible there and you can download them from there to any device one at a time. More convenient would be to turn on iCloud Photo Library on another device and let them all download automatically.

If you didn't have iCloud Photo Library turned on but did have Settings > iCloud > Backup turned on, the photos will not be visible at https://www.icloud.com/

You can however use the backup to restore to another iOS device: Transfer content from an iPhone, iPad, or iPod touch to a new device - Apple Support