Pain in the Ass and it costs. With road bikes, Especially tri bikes, Fit is everything. I ride as my primary transportation, but I'm the boardshorts under cargo shorts, skate shoes, 50lb backpack adventure type rider on a Specialized Allez steel frame roadbike. I'm a bicycle mechanic part time privately but used to work at a road oriented shop.
Here's what you do. Just go to a bike shop that carries road bikes and tell them you're looking for somethinglow end but not sure what size you are. They'll size you up super fast...shoot the **** with them a bit and remember the number. Most likely a 56 but Maybe a 58 if you've got long legs. See what they say.
I deal in used bikes and will keep my eye out.
PM me any messages after this and we'll talk about board trades.
hey - congrats on getting into road biking, I got addicted to the pain/reward aspects of riding about a year and a half ago. Im 6'3 200lb and ride a 59 cm. at 6' your probably 56-58, depending on leg length/inseam. good luck!