Plastic works also has misc. pieces that you can buy for like $5 a piece - you'll just have to bring a measuring tape to check if the pieces they have are the right size.
They are right off I-95 in New Ro, so, easy to get to for all.
They also sell their own version of Weldon (very watery one) and it works awesome, I used this for al my acrylic building (and have some left)