Come up with a design and try to stick with it. I bought some items that I thought would look good but when I put my costume together they just didn't work.
Along with the thrift stores, look on E-Bay (with parents permission of course). The pocket watch you can get at the con for $40.00 is often available for $20.00 on line.
vampricyode: I bought some pocket watch chains on e-bay. They might work for your knife. If one chain is not long enough, buy two and hook them together. Thats how my pocket watch got a full fob lengh.