I still use a ton of 1-bit clip art, which I prefer due to its ability to duplicate nicely on black and white copiers.
The collections I typically pull from include:
1. WetPaint by Dubl-Click Software. I have two libraries of WetPaint (original and All the People). There are about a dozen of these libraries out there but all are tough to find. Dubl-Click Software has been defunct for some time but was one of the pioneers of digital distribution. (I bought one of my two sets that way, the other is on three 3.5" 800K floppy disks). Each set includes tons of bitmapped images. Some have themes while others are general collections. The real gem is "The Wet Set", which had all of Dubl-Click's libraries on CD. Note that some images were from an earlier collection called Mac the Knife (from Miles Computing).
2. HyperCard's "Art Bits" stack. The content tended to change by HyperCard version, but most included several categories of images. It's not as extensive as WetPaint but has a lot of nice selections, including some that can easily be placed into a HyperCard stack as interface elements. There was an additional collection included with the HyperCard for Education set for classroom use.
3. The images from the original "The Print Shop" (the monochrome version, not "Deluxe"). The graphics come in four sizes (small, large, horizontal full panel, and vertical full panel) and can easily be exported via clipboard or a MacPaint document. (It is also possible to import MacPaint files to Print Shop). The images aren't categorized, but you'll be able to find just about anything you want.
4. Claris images, pre-ClarisWorks. These were sold sometime in the late 80s/early 90s when MacDraw II was current. The selection isn't huge, but it's got some nice stuff. It's also nearly impossible to find--I think I may have one of the only surviving copies of these disks.
5. Cairo symbols, bitmapped. This can be done by typing in the Cairo font in a paint program, which turns them into bitmaps. I think some of the earlier versions of HyperCard had Cairo symbols in their Art Bits stacks.
6. Free/public domain bitmapped images from old BMUG collections.