Of course it depends what you're looking for, but by and large you probably want either more wide angle, in which case probably the 11-22mm lens (the 7-14mm is even wider but it's expensive) or more tele. If the latter the second kit lens that they sometimes sold with the 14-45mm, the 40-150mm lens, is actually a good lens and bit faster than the 14-45 that you have. It is pretty widely available used. Most digital Zuiko lenses are hard to find used because they're quite new. The upgraded standard lens, the 14-54 does not make a lot of sense for you as a second lens unless you need more speed. It's also tougher but optically about the only difference I can see is that the 14-45 is quite a bit more prone to ghosting in some conditions.
My sense is that you would look not only for used lenses but also Sigma lenses which are becoming more and more available in the Olympus mount. For some reason B&H does not carry the 30mm f/1.4 - Adorama does - but they carry a bunch of others. I would suggest B&H (www.bhphotovideo.com) first. For used ones keh (www.keh.com) is worth trying for the 40-150. I did find the 1.4 teleconverter (not a cheap lens) at KEH in near-new shape. (This converter does not work on the 14-45 or 40-150 lenses; or rather, Olympus doesn't recommend this; I don't know what would happen!) You could try eBay for the 40-150 but you sure need to use caution there. I have seen various Olympus flash units on eBay and again caution is needed. Personally I dislike flash and would want the really pricey unit with a bracket if I added it; given how seldom I'd use it I haven't sprung for the hundreds it would take. If you want one of the smaller Oly units you'll have luck; I don't know what to recommend in third party flashes.
For what it's worth, based on a lot of film, digital (camera and "darkroom") acquisitions in the past couple years, I'd use KEH for used, eBay only for obsure items not found elsewhere, B&H if I need to use a far-away store (though Adorama and Abes of Maine are OK, just more pushy and even B&H misled me about the properties of one item when I asked about it before buying) - and a local specialty store (not, repeat not, a large chain) whenever possible. You might not have one near you, but if you do I believe you will absolutely more than make up any difference in price with better advice, intelligence about forthcoming products, and after-sales service.

thnk u

You're welcome. Thought I should clarify one thing: by national chains I didn't mean to include Calumet. Also, there's a home page for all Four Thirds products; the lens page is: http://www.four-thirds.org/en/products/lense.html.