Kris,
Spacers ... Hubcentric, settle for nothing else. I ran 1" in the rear for 6 years on my 98. Anything wider .. I'd measure to ensure proper clearance.
Struts/SPRINGs .. I'm a suspension pkg guy myself, I'd look at Bilstein, Tokico or FRPP's and Eibach or HR offerings for a firmer/sportier ride.
Where? There are a lot of places to order from, online or parts stores. It would really depend on what you prefer. STEEDA's stuff has been good in the past and for your MY proven.
Kinda like asking which do you prefer BLONDE, BRUNETTE or REDHEAD? ... Opinions will vary.
There are cheaper and more inexpensive products too. If you have an opprotunity to ride in a modified mustang and like the feel ... check out their set-up. Great way to baseline expectations.