Here are two sites and if you register you get weekly freebies from both. Just be sure to read the copy write laws for EACH set of clipart! You can also do a search for 'sets of clipart for sale' and find many sites! Good Luck
Good you mentioned copyright. I have visited some sites that sell beautiful clipart at reasonable prices but their terms and conditions specificly exclude the use of them for digitising. Others only allow personal use. So check before you buy!