Hi Jeannie!
I'm not so sure what you mean: You have a tub in the midst of your outside garden and want it to be replaced with a shower outside, lol?
Hmm, in either case, you'll need a plumber, I suppose. So, with that said, you should contact local plumbers in your are for a free consultation and estimate, as the cost of material and labor greatly varies by location.
Incidentally, for your convenience, here's a handyman site I use often, which has a section devoted to plumbing projects, where you'll find a text-link ad towards the middle of the page that offers up to four (4) Free estimates from local plumbers (it's the text-link ad that reads, "Find Local Plumbing Services. Get Up to 4 Quotes Now...Free"):
http://yourhandymanzone.com/Your_Handyman_Zone_How_To_Pages_Home_Improvement_Zone_Plumbing.htm
Yeah, so your best bet is to get estimates from local contractors in your area and go from there in discerning the feasability of replacing your tub with a shower. Good luck!
