I replaced my 2028 Primer button O-ring, although Eheim refuses to sell them. But that big O-ring is indeed available elsewhere and replacement was rather easy.
Available where, you ask? Let me quote from this link found in Cichlid Forum
<quote>
...from theoringstore.
The sizes you will need are 4.5mm X 92mm for the motor/primer and a 2.5mm X 12mm for the flow control indicator.
This is what I ordered:
N4.50X092 Metric Buna-N 70 O-ring$2.80
2.5mm X 12mm Metric Buna-N 70 O-ring$0.22
</quote>