i've heard that the dr pepper taste is very close to cola extract with almond extract added, and maybe a little cherry extract as well. you could use that as a starting point and make small test batches. experimenting is fun
as for a ready-to-use dr pepper extract clone, i've never heard of one.... there are lots of ready-to-use postmix syrup clones, sam's club is probably the cheapest but it uses hfcs as the sweetener... blecch!