has always been a great source for both Mason Bees and Tubes. Buy their tubes, don't bother with the bee blocks as I find these don't work as good.
It's easy enough to make your own nesting block. There are droves of videos showing "how easy" it is by drilling holes into wood. The trouble is these bees are really interested in holes that run longer than 4 inches. Not that they won't use the 4 inch tube, it's just they'd really like around 7 to 12 inches a lot better. Drilling holes in wood takes forever. I've found it's easier to dremel grooves into a sheet of wood, and lay another sheet of wood on top. In this way they can be stacked on top of one another.
As for buying bees. This really isn't as necessary as it might seem. In nature they're nesting in old plant stems, last year's raspberry canes, and small holes in trees. Often the former damage left behind from a wood boring beetle grub will be used as a nest the following year. If there's a patch of forest anywhere nearby you can be sure that there's a species of these bees around.
Early in the season "Mason Bees" are out. They're great pollinators of spring blooming plants, but are otherwise done with their life cycle after 6 weeks. New mason bee species emerge over the summer but overall the majority of them are only active for spring blooming crops. So they're great for your strawberries and apples but no so much for squashes and tomatoes.
As late spring and summer roll in we get "Leaf Cutter Bees" emerge. Like mason bees they use per-existing holes but rather than plugging it up with mud, they'll use leaf and flower petal cuttings. When present they can sometimes pick on a favorite plant leaving lots of dime-sized holes punched along the edges of leaves. This doesn't really hurt the plant but it's something to take into account. I've ready they have a preference for Cherry trees, but I've had them go for our Eastern Redbud sometimes. Some leaf cutter bees also nest underground and simply need loose soil.
Well into the summer is when "Mason Wasps" occur. These live just like mason bees, using mud to plug up their holes, but rather than doing any pollination they're more predatory. They're beneficial but don't really pollinate anything.