How can I used double-faced cards in casual play without checklist cards?
A checklist card is simply a tournament-sanctioned proxy with a specialized purpose. In casual play, you could make a proxy more-or-less any way you want. Writing with a marker on a basic land is probably the most common custom.

I recommend including the mana cost as well as the card's name, so you can quickly tell whether you are able to cast it without having to look over at your double-faced cards outside of play.



If you wish to get some more of the official checklist cards, you can buy them online for about 10 cents each through a major singles dealer like Star City Games or Channel Fireball (since they resell the token cards that come in boosters).

Note that Innistrad and Dark Ascension have their own separate checklist cards.