And there are lots of plant sources that are missing or insufficient in essential amino acids. Beans, for example, are lacking in methionine, which is why corn and beans are eaten together in some traditional diets.
Besides, you probably don't want a lot of methionine in your diet (it's linked to feeding cancer) [1..7].
Further, you're recycling the tired myth that "protein comibing" is necessary. Our body maintains pools of free amino acids by dumping protein into the digestive tract, which are broken down and reassembled, i.e. it does the protein combining for us, it's not necessary to explicitly do it through meals if you're otherwise eating a varied diet that sufficient in calories.
[1] http://www.ncbi.nlm.nih.gov/pubmed/22171665 [2] http://www.ncbi.nlm.nih.gov/pubmed/18789600 [3] http://www.ncbi.nlm.nih.gov/pubmed/15955547 [4] http://www.ncbi.nlm.nih.gov/pubmed/18252204 [5] http://www.ncbi.nlm.nih.gov/pubmed/22342103 [6] http://www.ncbi.nlm.nih.gov/pubmed/11603655 [7] http://www.ncbi.nlm.nih.gov/pubmed/14585259