Yes, it's a simple mod - Mikey Perrin is usually the best source for armrests. Last time I got one from him it came with a caveat that it wasn't resold on ebay as they were going for much more. Don't look to pay more than £45-£50 + p&p.
The securing bracket is imperative - don't buy one without the fitting kit.
i bought a cloth one off ebay but someone off here was sellin it and i paid £16+p+p and then i re-covered it in leather(not real leather tho) but it looks really good and is really easy to fit!! i'd recommend getting one to anyone
You might find that the seat rake adjustment dials are a bit tricky to adjust with the armrest installed, however its not impossible, and its a good looking and easy to fit modification which is worth doing