[IKE]

With a spring like that, you will need something to stabalize it to kep it from buckling. You can often stack springs on a shaft or inside a reasonably good fitting shaft (you need some room for growth, but not enough to allow for a buckle). I would recommend that shaft if it is possible. When stacking the springs, if they do not have a stackable seat, you can put a washer between them and stack them that way.
Remember your spring calcs (or look them up). In a stack, 2 30lbs/in. springs stacked in series turn into a 2x long 15 lbs/in spring. I think you may find some of your criteria overconstrain what you can get in springs. For example, buying 18 (or 3 packs) of this spring from McMaster:
Steel Compression Spring Zinc-Pltd Music Wire, 2.00" L,.970" OD,.125" Wire
And stacking them around a telescoping tubes would result in a 36" long spring, but the rate would be about 5.5 lbs/in and this is using wire that is more than 2X your required wire diameter.
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I am not sure about your application, but you might want to look into a "Gas Strut" They are nice compression springs that do not buckle and can be found in about 36" length with around 16" of stroke. The ones I am seeing on McMaster Carr are around 25 lbs. of force. This is a bit higher than you were looking for, but I think it might be better than a stack of springs and washers on telescoping tubes.