The source for replacement parts depends on the kit manufacturer. (If you buy from a local dealer you may be able to talk him into opening another kit and scrounging the part you need, but it's doubtful. And internet/mail order firms usually don't offer such service - though there may be exceptions.) In most cases you'll probably have to deal directly with the manufacturer.
The big companies, such as Revell and Airfix, seem to have been doing pretty well in the customer service department lately. (I ordered a replacement part from Airfix a while back and got it - despite the transatlantic delivery problem - in a couple of weeks, along with a nice covering letter from a genuine human being.) If I remember correctly, Revell/Monogram's website has an "Order Replacement Parts" function.
The problem you're likely to run into is that so many of the best plastic sailing ship kits are out of production. None of the big companies has released a genuinely new one in about twenty years. Revell, for instance, used to have more than a dozen sailing ships in its catalog - including some of the most sophisticated plastic kits ever made. The current Revell/Monogram catalog contains two sailing ships: two versions of the U.S.S.
Constitution, one fifty years old and the other forty. The Revell-Germany catalog (for which there's a link on the Revell/Monogram website) has a few more - but I have no idea how good the German branch of the company is at parts replacement.
There are four plastic H.M.S.
Victory kits currently on the market. The smallest is from Airfix. It's about five inches long, and at least forty-five years old - hard to take seriously as a scale model. Next in size is the Revell kit, originally released in 1959. (It's not being marketed by Revell/Monogram, but is in the current Revell-Germany catalog; decent American hobby shops stock it, or can get it via special order.) It's a nice kit for its age and size (about 18" long). In my personal opinion it is, in fact, a more accurate and marginally more detailed kit than the slightly larger Airfix version, which dates from the late 1960s and is about 20" long. That kit has some things to recommend it, but to my eye it's wrecked by the fact that its bow is seriously distorted.
The granddaddy of all plastic
Victory kits is the 1/100-scale one originally issued by Heller, of France, in about 1977. It's a little over 3' long, and arguably the most sophisticated kit yet produced by the plastic kit industry. It does have some problems in terms of accuracy, but those problems are pretty minor. (We've discussed some of them - notably the entry ports, the height of the forecastle bulwarks, and the absence of parrels for the yards - earlier in this thread.)
The kit's biggest weakness, perhaps, is an unbelievably wretched set of English-language instructions. It would be difficult if not impossible to build this kit without recourse to outside references. (Fortunately there are quite a few of them.) It is, in any case, a challenging project - in terms of difficulty, time, and expense. To do it right would require quite a few dollars' worth of aftermarket parts (blocks, deadeyes, etc.) or a great deal of scratchbuilding. But those who've finished it will tell you the results are worth it. The kit has recently been reissued under the Airfix label. (Airfix and Heller merged a few years ago.)
If you're relatively new to sailing ship models and want to try a
Victory, my recommendation, for what little it's worth, is to start with the Revell kit. It's a basically sound, accurate reproduction of a beautiful and extremely important ship.
Hope this helps a little. Good luck.