Liveaquaria.com is a high quality, fair priced online store, and so is bluezooaquatics.com. Both have gotten great reviews. However, with any online store usually if your purchase is under $175 (for liveaquaria it needs to be over $200) then shipping is $30-40! You might want to reserve the next convict tang at your lfs
if you're able to, unless you are willing to pay that money. Tangs are more prone to ick than most, so I'd say you should quarantine it so your other fish is not effected, b/c if they were, you'd have to quarantine all of them. Drip acclimation is best..so acclimate them to the QT tank and put them in the QT and moniter/treat if needed for a month.