If you are removing the 3-D metal tank badges it is easy. They are held on by double sided tape. You can warm them up with a hair dryer. Then use dental floss to "slice / cut" trough the tape (take your time). Once the badge is off you will have some residue left. You could use prep-sol or other mild cleaners on a cotton rag to help remove the residue. I would wash the tank thoroughly after using any non-standard cleaner on it. The process is pretty easy and just takes a few minutes to do it right.
I have had VERY good luck getting adhesive off my tank using a clean rag soaked in WD40. I just placed the rag over the spot and soaked it with WD40. After a bit, ALL the adhesive wiped off. I was pleasantly surprised at how easily it came off.
There's a huge number of badges on e bay.You should be able to find what you want by typing in something like:Harley tank badges on ebay or:Harley tank emblems on e bay.