What To Look For
When purchasing something in SL it's important to consider three factors:
1) Necessary Features
It's important to make sure that what you're buying actually does what you want it to do.
Quality Per Dollar
Just because something is more expensive doesn't mean it's better.
Certain products essentially require certain permissions.
A nocopy/nomod avatar defeats the purpose of looking unique. Also, if an avatar is nocopy, you can't create a backup of it, so if something goes wrong that's it. Game over. I recommend never purchasing a nocopy/nomod avatar.
A nocopy vehicle is just asking for trouble. While an attachment is less likely to get "lost," a vehicle will get lost early and often. Banlines, crashes, griefers, they can all cause your boat to get lost. Being able to create as many copies as you want is essential.
Where To Look
There are places inside SL where you can find things to buy, but there are also places outside. It's important to explore all the options available to make sure you're buying the best product.
The SL Classified search is one of the best ways to find things in Second Life. However, it's important to remember that they're sorted by how much people paid for them, and quality items may not be at the top.
To bring up the search window, you can press Ctrl-F or use the top menu bar by clicking Edit -> Search.
You may need to click the "Classifieds" tab if it isn't already selected.
Since our goal is to find awesome toys, we want to limit the category to "Shopping." To the right of the search input box at the top, click on the category dropdown and select "Shopping."
Note: Sometimes people put things in the wrong categories. If you're not finding what you want, you can try selecting "Any Category" and see if it turns up what you're looking for. Also, you can make sure "Include Mature content" is checked to possibly return more results.
The rest is easy! Just enter your search term and either hit the Enter key or click "Search." A listing will appear on the left. Click on one to get more info and a picture on the right. After you find what you want, click "Teleport" to travel to the store. Your classified search is saved until you log out, so to visit the next store you just have to bring up the search window by clicking Ctrl-F or selecting Edit -> Search again.
Note: Due to sim restrictions, some stores cannot let you teleport straight to them, but instead teleport you to a "General Teleport Pad" for the entire sim. You'll know this has happened if a glowing red marker is off in the distance. Just head towards the marker and you'll find what you're looking for. Also, occasionally a store will shut down without updating their classifieds. If you can't find the store, even at the red marker, then it has probably passed on.
There are other tabs you can search under in Second Life, too. The second most useful for finding products would be the "Places" search. To limit results, choose "Shopping" from the dropdown as the type of place you're looking for. If you don't get the results you were hoping for, leave it set to "All" and see if someone misfiled their search
SL Exchange or OnRez are store websites that is run by SL Residents, not Linden Labs. It has a much more robust search engine than the internal Second Life searches, but only has products people have listed on the sites. Occasionally they're priced higher than the in-world stores because the website takes a small cut.