Do you know any neighbours nearby who have ponds? If so, ask them if they have any surplus frogspawn, or put a note up on your front garden gate asking for some. If your spawn then hatches, hopefully the baby frogs will grow up in your garden and return to your pond in subsequent years to breed. But if you have fish in your pond it's quite likely they will eat the tadpoles so keeping one pond without fish is probably a better long term way of attracting and keeping them.
Also, is your garden totally enclosed by walls or fencing? If so, and there are no holes and gaps low down in the fencing near the earth through which frogs can squeeze, it will be difficult for them to get into your garden, feed and breed. We only have two very miniscule ponds in our garden, but an adjoining neighbour, and one at the back of us both have ponds too, so every spring I try and make sure there are some small gaps at the bottom of our communal fences so that frogs can migrate from their gardens into ours, as both neighbours' gardens have more undergrowth than ours where the frogs are likely to hibernate over the winter.