Where could I find a huge harley-davidson patch for a blanket?

The best places to find Harley patches is at Harley Davidson shops.

As far as buying one big enough for a blanket,I'm not sure they sell them.

How about making one?

For the Harley insignia all you need is orange and black material.

You can go to the local Harley shop and ask for one of the biggest bags they have with an insignia on it.

Go to your local material store and purchase a small amount of the black and orange material and some iron on backing.

Then apply a iron on backing(follow instructions) to the material, trace the pattern on the backing(backwards).Cut out a little bigger so you have room to hem and stitch together.Then all that's left is to iron it on to the blanket carefully and sew it on for reinforcement.

You'll have many choices at the material store for what kind of material you prefer to use.And to get the iron on backing.

Being the blanket is fleece, I think a nice satin type material would go nice.

Hope this helps :))