Here are well-known custom software development companies and engineering centres in Sri Lanka (short list with their typical specialties and locations). I included a reliable source for each entry so you can check details.

1. WSO2 — API management, integration, identity & middleware; large engineering centre in Colombo. (ten.lk)
2. 99X Technology — software product engineering (Agile, ISV partnerships), strong in Europe/Nordics; Colombo-based. (srilankabusiness.com)
3. Zone24x7 — end-to-end product engineering, embedded systems, data science, IoT; Colombo R&D/engineering centre. (Zone24x7.com)
4. MillenniumIT ESP — enterprise platforms, systems integration, managed services and software development (headquartered/large operations in Colombo). (en.wikipedia.org)
5. Virtusa (Sri Lanka) — global IT services & engineering, digital transformation and application development; major Colombo delivery centre. (Virtusa.com)
6. hSenid (hSenid Software Lanka) — enterprise/mobile applications, outsourcing & HR/telecom software; offices in Colombo (and other Sri Lankan cities). (hSenid.com)
7. Calcey Technologies — agile product engineering, web/mobile/cloud, UX & QA (development centre in Colombo). (calcey.com)
8. John Keells IT (JKIT) — enterprise solutions for travel/aviation, hospitality and related verticals; Colombo-based IT arm of John Keells group. (en.wikipedia.org)
9. (If you need larger/global consultancies with Sri Lanka delivery) Virtusa (listed above) and other global firms have sizable Sri Lanka centres — use the firm page above for contact and services. (Virtusa.com)

Notes and next steps

• This list is a concise starting point; there are many additional boutique shops and specialist teams in Sri Lanka (e.g., companies focused on fintech, healthcare, embedded systems and QA) — I can expand the list to 20+ firms if you’d like.
• If you tell me your priorities (budget, tech stack — e.g., Java/.NET/Node/Python, domain: fintech, healthcare, e-commerce, or whether you prefer product engineering vs staff augmentation vs fixed‑price projects), I can shortlist 3–5 firms that best match and include contact pages and suggested engagement questions.

Would you like me to produce a short RFP template you can send to 3–5 of these firms, or shortlist by a specific tech/domain or budget?