Quite a few years ago there was an organisation who would put local people with ponds who wanted frogspawn in touch with people who had ponds with more frogspawn than they needed.
It worked very well but I wouldn't do it now as the frog population locally (hants) has been badly hit by the redleg frog virus. The frogs left are resistant to the strain they have encountered but, of course potentially carry the virus so populations really shouldn't be moved around.
In the past wjhen it was legal I used to collect frogspawn, raise the frogs in a big tank in the garden and then release the frogs back into the wild. It never seemed difficult. In the wild most frogspawn doesn't end up as a living adult frog, frogs, tadpoles and spawn are an important part of the foodchain, which I guess is another reason not to take them from the wild.