What I have done is to invest in a fairly large selection of parts. I have one of those WalMart 4-drawer plastic storage units that has 12"x12"x2-1/2" drawers.
The top drawer contains freight car trucks of all sorts and sized (I just ordered 9 more various trucks from Tahoe). The second drawer contains roofs, running boards and underframes (varous sources), the third drawer contains doors, ends and bagged detail parts (ladders, tack board, and other side & end details), the fourth drawer is random parts that I did not use when building models (it contains anything you can think of).
I do have two other containers. One is about the size of a Plano 3700 tackle box, but has no dividers. That contains all brake parts (AB, ABD, KC and KD), brake wheels and other details such as brake rods, retainer valves, brake platforms, etc.
The last container is a small divided tackle box, the kind you store hooks in. One compartment has eye bolts, one angle grabs (the type on the roof corners - also usable for some cabooses), one has curved caboose side grabs, one has straight grab irons from 11" to 36" long (scale inches), one has drop grabs (mainly 19" but others too) and one has coupler cut levers.
Now before you ask how much did that all cost, remember that I don't smoke or drink, so the money someone would waste on cigarettes or booze go into parts and these parts have been collected over a 30-year period.
The best place to get started getting these parts is the Detail Associates section of the Walthers catalog, Tichy trains and, hidden on the Atlas website is all the old Branchline Trains details parts.
Also, go to train shows. You have a great one in JAX each year, and look through the "junk" vendors. You can buy lots of parts and even whole kits dirt cheap. Undecorated kits are usually plentiful and cost less than decorated ones. So even if I don't need a C&BT shops boxcar today, for $2 or $3, I'll grab it for future projects or even parts. When Branchline sold their RR cars to Atlas, I bought every undecorated Branchline kit I could get my hands on (probably have 10 or 15). Finaly, as I alluded above, I never throw out parts (except plastic wheels and hook-horn couplers) but toss them in the parts drawer. That is the first place I go for detail parts for a model I am working on.
P.S. Almost forgot. Have two other Plano 3700 boxes. One full of Kadee, P2k and Intermountain wheel sets and the other is full of Kadee couplers, although I mostly use the "Scale" whisker couplers these days, but I have at least a few pairs of each model they make if I have a difficult fit (e.g., front coupler on a steam engine).