Erm, tell them to do one!
Why on earth would they want proof that you haven't been working the last 8 months?
At which point, they may well tell the potential employee (altich88, in this case) to "do one" over his ex-new job. Remember that it's up to an employer who they hire, and don't hire, provided they don't fall foul of discrimination legislation in the process. As this is not one of the issues protected by anti-discrimination laws, they'll probably just withdraw the offer if told, politely or otherwise, to do one.
That said, I agree it's rather offensive asking for these and, personally, I would be wondering if I wanted to work for them. But, after being out of work for 8 months, that may be a line altich doesn't feel inclined to take.
@altich .... have they put any stipulations on the form? Are copies acceptable or must these be originals? If originals, then short of hand delivery, nothing is entirely safe. Recorded delivery is relatively safe, as is a decent courier, but both could go wrong. Can they be hand-delivered to a senior manager at your office, who then copies them, signs them and forwards them to HR.
Also, can transaction data be blanked out? If so, consider what to obscure.
Can HR provide, and I know it's a bit last century, a point-to-point fax number? Failing that, as others have suggested, some form of encrypted file, and the key under a different delivery method is probably the best bet.
Also, how do they know that the statements you provide are your only bank accounts? Or, for that matter, that you weren't paid in cash for the last 8 months?