"whats the best website to shop from?"
First of all, thank you for your service. Without guys like you, we wouldn't have the country we have.
You'll get plenty of favorite websites from some guys on here. I, too, have been successful with shopping for parts on line. I'll leave that up to the others on the board. I used to have a 1985 OMC 6-cylinder that I owned for 28 years. Since OMC had gone out of business, parts were getting a bit hard to find, but I always found a way. Many companies sell aftermarket parts, and that is a pretty good source. Others will give stories of how the aftermarket parts are inferior to the OEM parts.
One of the things I strive to find for any boat I have owned is a manufacturer's manual of some sort. Many times, replacement parts lists are available. In today's world, Google is a buddy. If you have any numbers, Model number, outdrive number, engine number, year, model, etc., just add the part by part description and Google what you want. For example, in my current Volvo Penta with a General Motors 5.7 L engine, i found that the oil filter was not the normal screw-on canister type. The brand name was Mann. I took it to my local independent parts house, the owner cross-referenced it and found the equivalent Mann part number for me. As it turned out, it was $6 less expensive that the VP filter of the same part number.
Other places to look is at the companies that specialize in marine parts. They are usually very helpful and have parts lists on their websites. And, once in awhile, they have pdf files for their parts.
Good luck. this is just a start. I am sure when the rest of the guys chime in, they will have some great input for you.