Ok,, you have clarified your desires a bit more.
The Blackhawk in 357 w/o a 9mm cylinder can accept the extra cylinder and work,,, IF the following is done.
First,, the OAL has to fit. When those of us buy an extra cylinder,, we look for one that is the same OAL or slightly longer, so it can be fitted. Next,,, if you do not know how to check to see if the timing is correct for your gun & the new cylinder,, then a gunsmith is needed.
But,, the best thing is to go ahead & have a gunshop, (or do an internet search & buy there) get you a factory convertable model.
As for ammo,, local places like wally world are hit & miss in the caliber & selection. Gun shops carry more calibers & such,, but are not capable of buying in the bulk that wally does.
Ammo prices have risen sharply in the last few years due to the increased costs of materials,, AND demand.
Internet shopping like Terry T has mentioned is one way to cut costs.
However,,, reloading is THE way to go. By becoming a reloader,, you can buy components in bulk and really save money on ammo. (Check out our reloading section and the "stickys" there to assist you.) By reloading,, you can maximize your range time with more shooting.

Plus,, look around for other shooting ranges that aren't as expensive either.
Just a few more thoughts.