I bought a cheap one off eBay ages ago - usually around £5-10 including shipping. -
https://www.ebay.co.uk/itm/Home-Char...EAAOSwHQ9WU13A
From what I understand, the USB charge rate is slower than the mains plug, which gives the battery a soft charge to start from near empty. Obviously it's never wise to leave it go so low, but that's a trick I've learned on my 505's