Have a look on free FB sites in your area, people are always getting rid of party bits. Failing that DIY kits etc on Amazon are pretty cheap
Shein has really cute ones for less than £20, I got my sons safari theme decorations there
Home Bargains do balloon arch kits for around £5
@Molly was it an arch with balloons did it come with the actual arch bit ?
You can get a balloon arch kit from Amazon for like £20 with arch and balloons
Did you want the arch for photos or just overall decoration? I think maybe adding some coloured ribbon to normal balloons could make them more impactful? Also ribbon in general can be quite cheap if you’re flexible and can be used again on gifts or decorations. Ill try and find some examples. Cx
Lots of really nice and cheap decorations on shein and Temu
depends what type of theme your going for, there’s a few things i’ve seen on vinted and things for birthdays, i saw one earlier that had all the decorations for a girls ‘berry first birthday’