Honestly, there's not much you can do for crosses without access to a pair of rings. Fortunately, you can get a pair online pretty cheap that you can hang from a tree branch, playground bar, or whatever at Welcome to Gymnastic Bodies Website
Wide arm pull-ups may help.
If you can access a set of rings, first train a solid support. Rings turned out, elbows locked, straight body, hold it completely stable. Once you can do this, there are two ways to attack the cross.
The first is to put your arms through the rings, so the rings are on your upper arm somewhere between your elbow and your shoulder and hold a cross like this. You can adjust the resistance by moving the rings closer and farther from your shoulders.
The second is to get a stretch band, put it between the rings, and put your feet on the band so that it holds some of the weight, and lower to a cross.
It is a very good idea to do some rotator cuff exercises if you are working crosses, as it is very very easy to injure the rotator cuff doing these.