I did find one by Google search that is sold on
Walmart.com. It is $25, but it is technically designed for the Baby Trend dougle jogger. Someone did ask if it fits other strollers, but no one answered the question. There were also several reviews that stated the cover ripped after a few uses.
I have found that with baby products the accessories for a product tend to be priced similar to the product itself. The Baby Jogger City Mini double retails for $400 IIRC, so $50 is what I'd expect unfortunately.
A google search also revealed a link to a listing on
Amazon of a place selling the BJCM double cover for $34.95, however, shipping is an additional $9.
Here is the link.
http://www.amazon.com/gp/offer-list...p?ie=UTF8&qid=1320859984&sr=8-1&condition=new
You mention only using it one or 2 days, but if you own the stroller the rain cover could get a lot more use. Even if it isn't raining it could also keep wind away from the children.
I figure you own the stroller because if renting, OSR charges $10 additional to the rental fee for the cover.
How old are the children? One alternative depending on their ages would be to have them wear ponchos. I wish my son was big enough for a poncho. He hated it when I put the stroller cover on.
I imagine with the Baby Jogger City Mini it's possible you may be able to use 2 individual ponchos, since it has two individual shades, but you might have to get creative with attaching it at the bottom. Honestly, the only time a poncho really worked on my son's stroller was if I had a short distance to walk, like just to get into a building and I fought the wind. If there is wind, you'd have to get kind of creative to keep it from blowing away if you planned to use it when the stroller is parked.
I wish you the best.