Nature reserves are usually private property, owned by Natural England, the RSPB, the Wildlife Trusts and so on, so removing anything from them without permission is theft. The pond where I got mine is on common land so this doesn't apply. Plus I shall be returning most of the tadpoles when they hatch and the rest of them when they've turned into toads so strictly speaking I've only borrowed them!
The best place to get some spawn is from a friend or neighbour's pond. Failing that you
can take some from common land but if you go that route please only take a
very small amount. Toads have been in serious decline in the UK in recent years and need our help.
Dave P.