There are 2 items I wanted to get for my avatar, but both appear to have been monthly items that I never see for sale. Is there any way to still get them?

There are some things you can do to attempt to obtain these items:

1. Create a thread in Item Auctions.
List what you're looking for, but don't really post what you are offering. The value of monthlies is generally really high because of the rarity and demand. Ask around for a value before offering. This might give you a better chance.

2. Wait for Black Friday/Cyber Monday.
Every year on Black Friday (US) the admins do a sale with past monthly avatar items. Last year they didn't include all of the monthlies, so some of the ones you are looking for may never come back. These deals are in the diamond shop, which means you need diamonds to purchase them. You can buy diamonds on the currency exchange with gold or you can buy them with real money.

Monthly avatar items may never go on sale in the diamond shop again. Overall don't get your hopes up too high because these are very rare (and precious) items you are seeking. Good luck!