...that way you won't buy a set of weights that are wrong for you!
Cans of food are good or fill a small bottle with water/sand/whatever - this is very useful as you can vary the weight.
You should be able to do 10-15 reps in 3 sets and end up with the muscles aching, but NOT killing - if in doubt do less with the Fibro. So if you find that easy (and remeber with the Fibro that your limits can change day-to-day) add more weight and if that's too hard use less weight.
Then if you want to, when you've worked out what feels good, you can weigh your homemade "weights" and go buy a set that size.