athinabean wrote: I am taking the GMAT in a little over a month and desperately need more practice tests, can anyone advise where I can buy/access more?

among other things, you can buy access to our practice tests, along with all the other online resources (= study organizer, exam results analysis tools, and so on). i'm not sure of the price, because i don't handle anything on the billing or purchase end of things, but you can contact us (800 576-gmat) and find out the pricing.

on mba.com there is also something called 'gmat focus', a collection of newfangled problems that seem to reflect the changing priorities of the math portion of the test. (gmat focus consists solely of math problems, at least for now.) those aren't practice tests - in fact, they aren't even practice test sections, since they consist of only 24 questions per 'test' - but they're still valuable. the price is a bit steep (iirc it's $75 for three tests, or $30 per individual test).

athinabean wrote:It would also be helpful to know why the huge discrepancy between my OG scores and the Kaplan/Manhattan scores?

it would be helpful to know the order in which / dates on which you took the tests, because, without chronological ordering, the results mean comparatively little.

also, remember that the standard error (approximately the same thing as 'standard deviation', if you don't know a lot about statistics) of the gmat is about 30 points, a fact that means that you should expect a fair amount of up-and-down in your scores. it would actually be quite remarkable if you amassed a list of scores WITHOUT the eventual occurrence of the sort of variation depicted here. (take a look at the wild fluctuations in the scores of moderator 'mayonnai5e' here, especially at the beginning of his trajectory.)
or:
some days you get all the lights red, other days you get them all green.
most days are somewhere in between.