A large number of slot card submissions are just for the card you obtain, typically the base level card while there may be additional higher level cards also available. If the casino will not allow giving out blanks of the higher levels when you ask, you might try asking if you can just take pictures of the other cards. This has worked quite well for me at times and gets us the images for the ChipGuide. Just be sure to take the front and back of each card just to be safe. There may be differences between the levels or the different level cards may be from different issues/printings.
Also, don't forget to ask for a few extra cards so you can share your new found treasures. Most casinos will give you 2 or 3 cards if you ask.