Two existing nouns with somewhat similar meaning are voluntarius and volo, volonis, both of which have meanings that can be glossed as “volunteer”. Nouns formed with the suffixes -arius and -o sometimes have definitions approximating those of agent nouns.
As for a neologistic -tor noun built on velle, the future participle voliturus mentioned by L&S and the Medieval Latin abstract noun volitio make the form “volitor” seem the most likely.
As per cmw♦'s comment, it's also a good idea to remember the present participle volens, which L&S explicitly says " = is qui vult, in the different meanings".