The directory http://www.buddhanet.info/wbd/
might also be useful...
I think that it might be useful to reflect on why you feel the need to be able to give money if you go on a retreat. For a start, there are other ways of giving (by volunteering to help out on future retreats, for example).
Also, I think that there is a huge psychological difference between feeling that you're paying for a service and realising that you are receiving a gift. If you feel that Ajahn X is instructing you because he is anticipating a donation, that's not really conducive to a good relationship. Furthermore, it would be very unhelpful to get to a difficult point in a retreat and be feeling that Ajahn X should be treating you better because, after all, you're actually paying for this.