I haven't got enough time to read through the other replies 'cause I'm about to go to work so sorry if I'm just repeating loads of stuff! But I had a similar problem at about your stage.. and now I think I've got it sorted, for now at least!
For now, I would invest in at least one bump band. They are AMAZING. You can get them from ebay. They basically go over your pants/jeans/whatever and around your belly so that you can wear your pre-pregnancy clothes for longer 'cause you don't have to fasten them up! I'm actually still doing this with my work trousers as I still haven't got round to buying some actual maternity trousers, which I'll definitely be doing in the next week or so. The only reason I can't still do it with my pre-pregnancy jeans now is that my hips/thighs/bum have expanded too and they won't even fit around there anymore
But that lasted aaaaages so definitely invest! You can get them for about £5/6 off ebay.
The next maternity item I bought was leggings. Again I got these off ebay super cheap! I got about 4/5 pairs in the size up from my normal size and these still fit me now and I think will do for a good few weeks.. after that I'll probably just order a couple more in a size or two up
They are one of my maternity necessities! So comfy for either wearing around the house or with a long top/dress.
So far I've got a few maternity tops but I've not really worn that many of them.. I've mainly just bought about 6/7 long vest tops in a UK size 16 (I'm normally size 12). These still comfortably fit me now and will do for a while. The only thing with these is that the straps are a liiiiittle bit long but it's not been that much of a problem for me, I just haven't got round to altering them yet. Also I can still wear a lot of my old tops 'cause my bump still isn't too big for them yet, and especially if you wear your bump band it doesn't really matter if it rides up a bit!
Last week I decided I got a bit sick of just wearing leggings all the time so I went out and bought myself a couple of pairs of maternity jeans. I got mine from Primark - on sale for £5 each!! I don't know if you're in the UK or not or if you've got a Primark nearby. Also not all Primarks have a maternity section so I'd ring up first and ask. But I know you can get them from Asda and other places too if you're on a budget. AND all the summer sales are on at the moment / over the next few weeks so go out and find some bargains! There's a good sale on at Mamas and Papas at the moment, I found a nice pair of jeans in there for about £10 but they were a bit on the small side. With the jeans that I bought I had to buy a size or 2 up from my normal size but I'm not sure if that's just 'cause it's Primark!
All in all, I think the best thing to do is try not to think too far into the future. Just buy for what you need now 'cause you could end up buying things you'll never use if you don't. Good luck!