Although I have about 5 preschool workbooks that I have picked up at various times, I don't always find worksheets that go with my theme. Sometimes it works out perfectly, like this coming week when I found a V worksheet in one of the workbooks related to Vegetables which is my theme. But only one workbook had a vegetable themed number worksheet and I wasn't happy with it.
On these occasions, I make my own worksheet. I started off using Microsoft Word because they have a great clip art gallery that includes photographs and I don't have to worry about copyright infringement like I would if I just downloaded photos from an internet search. (If you don't own Word, you can access their clip art gallery here.) But sometimes I don't find what I'm looking for. And I also like to use Google docs, which doesn't have a clip art gallery.
So I searched around and found WPClipart. As it says on their site: "WPClipart is an ever-growing collection of artwork for schoolkids and others that is free of copyright concerns as well as safe from inappropriate images." Unlike other "free" clipart sites, this one seems legitimate, without pop-ups. It has a fairly extensive gallery and although ad-supported, there is typically one ad per page so it is not overwhemling. It is easy to search images and browse by category.
To make worksheets using clipart, I might put groups of items on a page in varying quantities and have the kids circle the groups corresponding to whatever is our number of the day (see the worksheet which I will link to with the Vegetable theme after July 15th). This is an idea I got from Brain Quest Workbook: Pre-K. For the bug theme, I put various bugs on a sheet and had the kids identify which were NOT insects (based on number of legs) by circling them. For the same theme, I took black and white images of insects, edited them in Paint to remove the legs, and then had the kids draw 6 legs on them. Once, I made a worksheet that was a calender month (for April, I believe) because the number of the day was 7 and there aer 7 days in the week. Then on individual days, I put random clipart images and had the kids circle the ones that started with the letter of the day.
Another thing I like to use clipart for is visual displays, specifically clipart photographs. I like to have pictures for the kids to look at related to the theme. Pictures of clouds for the Cloud theme; pictures of fruits and vegetables for those themes. I like to either print them out so the kids can hold them, or to save printer ink, I'll create a doc which I then save them as a PDF (from Word or Google Docs) which I can load onto my Color Nook and hold up for the kids to see (I like using the Nook because it is small and easier to hold up than trying to show the images on a laptop sitting on a table. An iPad or other tablet would work just as well, but the Nook is what I have.)