I confess, I´ve always gravitated more towards Northern usage. So anyway, I found this note in Cymaeg, Cymrâg, Cymrêg … Cyflwyno´r Tafodieithoedd, p.90:
¨_ma ´na i isho_ : Ffurf lafar ar y mae arnaf eisiau , cystrawen sydd yn ildio mewn llawer man — o dan bwysau´r Saesneg, y mae´n degyg — i dwy isho.
This is a note to a transcript of a recording made in Ynys Môn!
Which means that the form I know is probably influenced by English, but then again the ´proper´ form suggested isn´t quite yours either, but close. Lit. ¨need is upon me¨ where you have ¨need is to/for me¨. Use of i often implies owning or belonging in a stronger or more permanent sense than ar.
Gareth King´s Modern Welsh Dictionary gives only variations of dwi isio in different tenses etc. (p.85)
T.J. Rhys Jones’ Teach Yourself Living Welsh (i.e. Cymraeg Byw) gives an extended (semi-literary?) form of the above Rydw i eisiau … (p43-4)
Dan L. James Cwrs Cymraeg Llafar : No blydi index , no idea what they use!
Going back to the days when (more or less) Literary Welsh was taught to beginners, the old Teach Yourself Welsh from 1960/65 has (p.115):
¨_Ar_ and its peraonal forms arnaf i, etc. can be used with eisiau (need) and the verb bod, to be, to express want, e.g.: Y mae eisiau bwyd ar y bachgen … ´The boy wants food´.¨ (+ similar e.g.s)
So essentially the older form with ar, but no mention of forms with i.
Morris Jones´ Welsh Grammar has nothing useful to say, but if you want to look go to p.414.
Wel, dyna´r cyfan dwi´n gallu dŵad o hyd atyn nhw yn fy llyfrau i. Someone else can look online if they like