What Are You Going To Be?
One source for ideas is the internet. Search with phrases like "Halloween costume for baby boy" and see what pops up. Search engines are meant to find the best page for the phrase you put in and it doesn't need to make sense for them to find it.
Another place to check is craft magazines. Some of these magazines have great suggestions in them like making old boy pirate garb into a renaissance look; or use a hand me down from years ago for your new toddler boy. A big boy ensemble this year is next year's outfit for a younger boy. These magazines also have great ideas for DIY opportunities out of things as simple as a sweat suit, some glue and felt. This is also where you can make a skeleton, such as a black jumpsuit with cut outs of bones. It makes a great skeleton and the materials can be found right around the house or at an arts and crafts store.
The cheapest place to find great ideas is around the house. An old suit can be dress up material for a boy; some clothes with fringe sewn on the sleeves can make a great American Indian; an old leotard can be a dance costume, or if it is too chilly for that at the Halloween party, use it as a base and add a sweatshirt and some foil to make a space alien.
All it takes, in the end, for that perfect boy pirate look or funny outfit is a little imagination and a little time. If you can't sew for your toddler boy, use glue to put it together. If you aren't sure just who shark boy or lava girl are, do a quick internet search for them. If the holiday party is tomorrow and he is still without a get-up, go to your closet and make your boy something.
It all really comes down to what you are willing to build and what needs to be bought. You might not be able to make white face paint, but you certainly can make a skeleton's body. So think about the costume you want, and think of how you can make it yourself. If it seems improbably, go online and look. There are tons of notions out there, not to mention commercial pages, and it never hurts to keep your options open.