Don't know where you're located, but certainly if you're anywhere near NYC, I'd recommend visiting Control in Williamsburg Brooklyn (http://www.ctrl-mod.com). They also do mail-order (including used gear), and from my experience they're the absolute best. Plenty of folks use Analogue Haven too. You won't find much a modular selection from Sweetwater - seems mostly Pittsburgh make, and while I've ordered plenty of studio stuff from them, I doubt they have the expertise to guide you towards your first rack.
Be sure to do your research BEFORE you buy - there's also tons of great info on Muffwigglers.com In particular it's good to check on a module maker's track record re: build quality, customer service and general customer satisfaction. Can't speak to buying used gear here, but have done well on Muff's b/s/t section (buy/sell/trade).
Good luck!
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