If you google "wellco boots" (or
http://www.wellco.com) you will find a company that makes many service-approved boots. You can also try ebay. I find great deals on boots at Einstein's, a Topeka surplus store. Bought a waterproof pair of desert boots by Wellco and an Edelweiss static 9.5mm rope for substantially less than the boots' retail.
Altama is the original company, but Wellco has license. They will have what is called "Panama soles" designed to grip well in mud. Lately they have stopped making them with a steel shank so they're even more comfortable. Originally the steel was designed to "protect" the foot from a mine blast but practical experience showed it just showered the foot with even more, secondary projectiles as the blast shattered it.
On a slight tangent, when I left the army in '93 I had 23 pair of boots! My time was in Special Forces, 3d Ranger Battalion and 2d Infantry Division in Korea. In SF and Rangers we were constantly being issued new uniforms and boots because our training tempo was so fierce that we destroyed them on a regular basis. I had about six sets of both black and green canvas jungle boots, multiple pairs of specialist mountain boots (on a mountain warfare team in Bavaria), urban combat boots (from Adidas, they finally went kaput last year), freefall parachutist boots (thick soles and no hooks to tangle in lines), dress "paratrooper" boots spit-shined to mirror gloss (and for inspection/dress green uniform wear ONLY!), etc
If I could lose 20lbs I'll have about 15 sets of BDU's to wear caving...
Dr K