As BasraBoy said...more information would be helpful here (i.e. - type of AR stock...collapsible or fixed, make/model, the features you are looking for or the addt'l height you need to achieve, etc.).
There are all sorts of aftermarket parts/accessories that will achieve which you are looking for, as well as many different DIY solutions. If setup correctly on an A2 buttstock, some of the stock packs like the Triad unit and others will work, but the biggest problem is getting the pack back far enough on the comb to allow the charging handle to operate properly. There are also "clamp on" units like the Delta cheek risers that will work too, but generally put the shooter WAY to high for flattop-mounted optics (as they were designed for using a scope mounted to a carry handle).
If you are talking about a collapsible stock, then there are products like the SAPR and Larue RISR that may work for you (again...hard to tell since we have NO idea what type of stock you are dealing with).
Then there are replacement stocks that will give you the adjustability you could want like the Magpul PRS stock (both adj cheek riser/comb and adjustable LOP).
Just got SAPR delivered and bolted on my UBR....yet to try it properly and adjust to fit but as ORD says....great fit/form/function. have also added Kasey's mod'ed strike plate for the MonoPod. Not as cheap as a PRS....but I like it!