Karlg;
The front sight on your gun is "staked" in place from below the slide with a punch and hammer (carefully!) or a special staking tool (recommended) then polished. There IS a difference between so called "Series 70 and Series 80" front sights as the newer ones use a thicker "tenon" (metal "leg" that goes inside the slide and is then staked or peened) than the older Series 70 and back guns...yours' included...you want the 70's type.
Rear sights for standard dovetails are the same size. Theoretically, you should always remove from left to right and insert from right to left so that if there IS a taper to the dovetail (as is the case with some earlier P35 FN's etc.) you will not widen the narrow end by driving the sight through it.
If you have a decent vise which can be padded with some leather or maybe heavy and not too hard plastic (I have used pieces of a thick leather belt) and some reasonable skill with hand tools, you should be able to accomplish this yourself if you are carteful.
Try Brownell's for sights. You can do a search for "1911 sights" and get several pages, but I think this is similar to what you are looking for:
www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=7637&title=FIXED+1911+HARDBALL+SIGHTS