Hi Peaches! Congrats on your upcoming (?) engagement. Yes, Hearts & Arrows stones can be found in stores. You might want to check with some of your local retailers to see what they have and what their financing options are. Some jewelers will still do layaway, while others have financing through companies like Wells Fargo.
As far as your question about the inclusions, it's entirely up to you and your fiance. Diamonds are completely different for each person. Some people (like myself) are very sensitive to color, and want a whiter stone, so they compromise a little on the clarity. Depending on the stone size you're looking at, you might be able to purchase a 1-1.5 ct with SI clarity that's eye-clean (meaning no inclusions are visible to the naked eye). Other people are really bothered by the idea of inclusions, so they sometimes sacrifice color to gain better clarity. There's no right or wrong answer--just what matters most to you.
I believe that well-cut stones can often appear brighter and sometimes more eye-clean, so you might want to compare different cuts of diamonds with similar characteristics to see whether that's true for you. The one thing that I've heard from many people, both customers and jewelers, is never to compromise on the cut. That doesn't always mean that you must buy the expensive "branded" cut (like Hearts & Arrows) but that you don't sacrifice on cut to get better color or clarity. Make sure to stick to well-known grading labs like GIA, AGS, and EGL also.
Best of luck to you!