Hi Kirsty
A company in New Zealand, Trig Instruments ( http://www.triginstruments.co.nz ), sells pocket magnifiers, which are great for botanical work. A friend and I both use their 10x pocket magnifier in the field.
For a greater range of options I suggest you try eBay (it's free to join). Searching under Loupe you will find both pocket magnifiers and hand held magifying lenses. Very cheap, but ask questions before buying.
Good luck.
Bill Campbell