Depends upon what is the objective of your experent.If you wish to study the response of plant on generation of ROS, then it can be any form of abiotic stress , may be salt stress, moisture stress or even waterlogging  and nutrient deficiencies. Generation of ROS is a kind of indication of stress induced defense response of the plant.

In animal cells substrates of monoamine oxidase will generate H2O2, octanoate will also do so. Antimycin A leads to H2O2 formation in mitos via superoxide and dismutation thereof. Outside the mitochondria you find exclusively H2O2. However, in general it is difficult to only generate H2O2, because H2O2 in turn  triggers superoxide formation and, in consequence other reactive oxygen species.

Dear Laura,

You can add direcly H2O2 which is stable enough to cross the membrane and analysed its effect. In parallel, you can add KI which is a scavenger of H2O2 as a negative control.

I hope that it will help

I agree about an addition of H2O2 - this approach is also seen in the literature, so you should find quite easily a publications for discussion.