Well, you won’t be able to buy a used one before hearing it unless you can find one within driving distance.
As far as websites, there’s Audiogon or US Audio Mart and its international affiliates, but you can’t go wrong with ebay. If the speaker doesn’t work right or even doesn’t meet the seller’s physical description (e.g. he said it was pristine but it's scuffed up), ebay will get you a refund if you file a complaint.
If you’re simply worried about the timbre-matching aspect, it’s just a risk you’ll have to take if you want to keep your Monitor 7-based system. But there’s no reason why another speaker from the same series won’t be a good timbre match, save perhaps bass response in a large version vs. a small one. If you ultimately don’t like it, you can simply resell it and probably won’t lose a dime, save shipping costs.
Regards,
Wayne A. Pflughaupt