My unit wouldn't issue maternity uniforms. I had to buy them. No one any where near here sold them so I bought a size larger than normal and wore that until I was too big to fit them. At that point, my unit allowed me to wear civvies.
That isn't the norm for a lot of units. I would check the local online yard sales and maybe craigs list to see if there are any listed. There is also a website called militaryuniformexchange.com. People buy, sell and trade uniforms there. You may be able to find something.