A couple of things mate...good and cheap dont really appear in the same sentance much! It depends what you class as 'cheap'.
Secondly, you get what you pay for!
There isnt a particular type of paintballs that will run well through your PMR. Pretty much anything will, as long as its good quality.
Give Jon or Chris at LiPS a ring mate. They will sort you out with paint and will be able to get it to you before Sunday